The Off-Key Bard
Technology

Why Do Different File Systems Exist, and What's the Difference Between NTFS, exFAT, APFS, and ext4?

Or: why the same USB drive can work perfectly on one computer and be unreadable on another. The files haven't vanished. The computer may simply not speak the drive's storage language.

Or: Why the Same USB Drive Can Work Perfectly on One Computer and Be Unreadable on Another

You buy a new external drive, plug it into your Windows PC, and everything works. You copy photos, videos, and backups onto it without a hiccup. Then you connect it to a Mac and discover you can read the files but can't save changes. Or maybe the drive works on your computer, yet your television or game console insists it must be formatted before use.

The drive isn't necessarily damaged. The files may be exactly where you left them. The problem is often the file system: the organizational language that decides how an operating system structures, stores, locates, and manages information on a storage volume.

A Drive Doesn't Know What a File Is

A hard drive or SSD provides addressable storage and nothing more. It has no concept of a photograph, a spreadsheet, or a folder named Vacation. Picture a warehouse with thousands of empty bins. The building supplies the space, but it won't tell you which bin holds the accounting records and which holds spare parts. You'd need numbering, a catalog, rules for filing, and records of where everything lives.

A file system is that system for digital storage. It supplies the structures that let an operating system create something recognizable as Vacation.jpg, place it in a folder, and find its contents later. Beyond that basic job, more advanced file systems add permissions, journaling, snapshots, encryption support, and protection against certain kinds of corruption.

Formatting is what puts those structures on a volume. Format a drive as NTFS and you create the structures Windows uses to manage NTFS. Format it as exFAT and you create a different arrangement. The hardware is identical, but the rules written on it change. That's why reformatting can fix a compatibility problem, and also why it can make existing files inaccessible. Never format a drive holding anything important unless it's backed up and you understand what the operation will do.

One distinction is worth making now. A partition is a region of a storage device, described by a partition table such as GPT or MBR, and a file system describes how files are organized inside that region. A drive can have a valid partition table and still contain a file system your operating system doesn't understand.

Why There Isn't Just One

Operating systems grew up with different priorities. Microsoft needed security features and reliable system volumes. Apple wanted storage tightly integrated with its hardware and increasingly flash-based devices. Linux needed flexible, dependable storage from laptops to servers. Removable media added a different requirement: compatibility across platforms. One file system can be built for rich permissions, another for simplicity and broad support, so no single format won.

NTFS: The Windows Workhorse

NTFS, the New Technology File System, arrived with Windows NT in the 1990s and is the standard for modern Windows system drives. It supports very large files and volumes, detailed security permissions, rich metadata, and features such as hard links, compression, and disk quotas.

At its center is the Master File Table, which holds records describing the files and directories on the volume: names, timestamps, security information, and references to where contents are stored. This is the machinery that lets Windows move a file between folders on the same volume by updating metadata instead of copying data, as the previous article explained.

NTFS also supports access-control lists, which define which users and groups may read, modify, or execute a given file. An administrator can let one employee read a shared document while letting another edit it, and protect system files from ordinary users. For a multiuser operating system, that's essential. And NTFS keeps a transaction log so that if power fails mid-update, it can use the log to restore the file system's structural consistency. That protects the organization of the volume, not necessarily the latest contents of every open document. Those are different promises.

NTFS's weakness is portability. Apple's support documentation says Disk Utility doesn't support the NTFS format, which a Mac "might be able to read from, but not write to." That's the classic experience: you can open a photo from the drive but can't save a new file onto it. Third-party software can add NTFS writing on a Mac, and options vary by macOS version, but if a drive shuttles between Windows and Mac, NTFS is rarely the simplest choice.

exFAT: Built for Carrying Things Around

exFAT is the successor to FAT32 in the FAT family. Microsoft's specification lists its design goals as retaining the simplicity of FAT-based file systems, enabling very large files and storage devices, and leaving room for future extension. It describes file size using 64 bits, which removes the old FAT32 ceiling, and it even includes an optional structure that flash-storage manufacturers can use to describe their media's erase-block and page sizes so that implementations can align data structures to the hardware.

The practical payoff is compatibility. Windows and macOS both read and write exFAT, and modern Linux supports it too. Apple groups it with MS-DOS (FAT) as a Windows-compatible format. Say you regularly move large videos between a PC and a Mac: NTFS gives the Mac write trouble, APFS gives Windows read trouble, and FAT32 chokes on big files. exFAT is the practical middle ground.

You do give things up. exFAT has no NTFS-style access-control lists, and it doesn't have NTFS's transaction log. The specification's baseline protection is a volume-dirty flag that is set before metadata is updated and cleared afterward, so a system can notice that a volume wasn't shut down cleanly. A transaction-safe extension called TexFAT exists, but the specification describes it as optional and used by Windows CE, and desktop systems shouldn't be assumed to rely on it. In practice that means yanking an exFAT drive in the middle of a write is risky. Proper ejection matters more than people think. For a USB stick carrying vacation photos, though, the simplicity is a feature, since there are fewer operating-system-specific security structures for another computer to interpret.

FAT32: The Format That Refuses to Retire

Before NTFS became standard, Microsoft systems commonly used the FAT family, and FAT32 became important in the Windows 95 era. Its superpower today is compatibility. Older televisions, cameras, game consoles, media players, firmware utilities, and embedded devices all understand it.

Its catch is a famous one. FAT32 can't store a single file of 4 GiB or larger, because its file-size field is 32 bits, so the largest size it can describe is one byte short of 4 GiB. Suppose you try to copy a 7 GB video to a 64 GB FAT32 stick with plenty of free space. The copy fails, not because the drive is full but because one file is too big for the format. Reformatting as exFAT removes that limit, as long as every device you care about supports exFAT.

FAT32 also survives in firmware. UEFI systems commonly rely on a FAT-based EFI System Partition for boot files, and some firmware-update tools expect a FAT32 USB drive. In a small pre-boot environment, compatibility beats sophistication.

APFS: Apple's Flash-Era File System

APFS, the Apple File System, replaced HFS Plus as the default on iOS 10.3 and later and on macOS High Sierra and later. Apple's developer documentation describes the new features: cloning, snapshots, space sharing, fast directory sizing, atomic safe-save, and sparse files. It was designed for modern flash storage instead of stretching an older format further.

Three ideas stand out. The first is cloning. Apple defines a clone as a copy of a file or directory that occupies no additional space on disk, available for fast, power-efficient copies on the same volume. The copy initially shares the original's blocks, and modifications are written elsewhere while the unmodified blocks stay shared. This ties back to the last article: a very fast copy isn't always data traveling at impossible speed. Sometimes the data never travels at all.

The second is snapshots, which preserve a point-in-time view of a volume without copying every byte. Because changed data is written elsewhere, the snapshot can keep access to the older state. That's handy for recovery and backup workflows, and snapshots can consume more space as data changes. A snapshot on the same drive is also not a backup. If the SSD fails, local snapshots go with it.

The third is space sharing. Apple's documentation explains that on many file systems, including HFS Plus, volume sizes are fixed when the device is partitioned, so free space can't move between them. APFS supports multiple volumes within one partition, and those volumes share the free space and can grow and shrink independently. That lets macOS keep separate system and data volumes without making you guess their final sizes in advance.

APFS is excellent for the ecosystem it was built for, and it isn't meant to be a universal interchange format. Plug an APFS-formatted external SSD into a Windows PC and Windows can usually see the physical device, but it generally has no native ability to interpret APFS. Third-party tools exist, with varying support. The data is likely fine. Windows just can't read the map. Apple's own guidance reflects this, listing exFAT and MS-DOS (FAT) as its Windows-compatible formats.

ext4: The Linux Mainstay

ext4, the fourth extended file system, is a long-time default across Linux distributions, though Linux supports many alternatives, including XFS and Btrfs. It's valued for maturity, large-file support, and predictable behavior.

Like other Unix-style systems, ext4 uses inodes, structures holding a file's metadata and references to where its contents are stored. Directory entries associate names with inode numbers. The kernel's ext4 documentation shows the scale involved: inodes are grouped per block group, the root directory is inode 2, and the journal itself lives in a reserved inode. That separation of name from data is why several hard-linked names can point to the same inode, and why renaming or moving within one ext4 volume can be a metadata change, the same idea as the NTFS move in the previous article, built from different parts.

ext4 also uses extents. Rather than listing every block of a large file individually, an extent describes a contiguous run of blocks, which keeps metadata compact. The kernel documentation notes that extent-mapped files have much higher size limits than the older block-mapped scheme: with extents the maximum file size runs from 4 TiB to 256 TiB depending on block size, versus 16 GiB to 4 TiB for block maps in the 1 KiB and 4 KiB cases. It also notes that a 64-bit feature lets a filesystem address up to 2^64 blocks.

And ext4 commonly uses journaling to keep the file system consistent after interrupted operations. In its usual ordered mode, metadata is journaled while data writes are ordered to land before the matching metadata commit, with other modes offering other tradeoffs. That is more nuanced than "ext4 journals every byte." It reduces certain risks without guaranteeing recovery of every recent application write.

Windows doesn't open ext4 drives in Explorer out of the box either, though setups such as the Windows Subsystem for Linux can provide access. The drive isn't broken. It carries a format Windows doesn't recognize.

Why a Drive Works Here and Not There

It helps to separate layers. USB describes how a device talks to the computer. It says nothing about what's stored on the device. A USB-C SSD could hold NTFS, exFAT, APFS, or ext4, and the same connector will work with every operating system while the file system remains readable by only some. A different cable won't make Windows understand APFS, and a faster port won't make an older television recognize exFAT.

Even for a fixed format, support varies by device: a television or console might accept exFAT, or only FAT32, or add its own rules about partition layout and capacity. Those limits come from the device's firmware, so check the actual destination instead of assuming from the connector or the capacity.

Smaller differences cause surprises too. Many Windows setups treat Report.txt and report.txt as the same name, while ext4 normally treats them as two files. And copying from NTFS to exFAT may not preserve NTFS-specific permissions, just as copying Linux files to a format without Unix permissions can lose ownership and executable bits. A successful copy doesn't prove every property of the original made the trip.

Two Warnings

Don't reformat just because the computer asks. If an external drive suddenly shows a message saying it must be formatted, the cause might be a file system the current OS doesn't understand, but it might also be corrupted metadata or a partition problem. Formatting doesn't diagnose anything. It prepares the volume for a new file system and can jeopardize what's on it. An unreadable drive isn't necessarily an empty one. First work out which system last used it and whether you need what's inside.

And remember that none of this replaces backups. Journaling on NTFS and ext4, and copy-on-write design on APFS, improve how changes are managed, but they don't protect against physical failure, accidental deletion, malware, or a power loss that interrupts an application's save. A backup on separate media addresses a different problem entirely.

So Which Should You Choose?

There's no universal answer, only a good fit for the job. For a Windows system drive or Windows-only external storage, NTFS. For an external drive moving between Windows and Mac, exFAT, one of the two formats Apple lists as Windows-compatible. For a modern Mac's internal storage, APFS. For a typical Linux installation, ext4. For legacy gadgets and firmware media, FAT32 when required. Always check the requirements of the device that will actually use the drive before formatting it.

Don't pick purely on speed, either. "Which is fastest?" depends on the workload, and an SSD's controller and flash often matter more than the gap between two file systems.

The Bard's Take

A file system is the organizational language that lets an operating system make sense of raw storage. The SSD doesn't understand folders, filenames, or permissions. It offers addressable space, and the file system defines how that space becomes information.

NTFS speaks fluent Windows, with permissions, journaling, and rich metadata. exFAT is the traveler's dialect, understood on most desktops and light on baggage. APFS is Apple's modern ecosystem language, built around cloning, snapshots, encryption, and shared space. ext4 is the mature workhorse of Linux. And FAT32 survives because being understood by almost everything can matter more than being clever.

None of them is universally superior. They solve different problems, which is why one external drive can work flawlessly on Windows yet turn read-only on a Mac, or look unreadable on a console while every byte is still intact. The operating system can talk to the hardware. It just may not read the organizational structures written on it.

Behind a simple double-click, the computer translates a name into metadata, follows references through directory structures, locates storage, checks permissions, and retrieves the data. Until you plug a drive into another machine and find that two computers don't share a storage language. The files haven't disappeared. The computer has just forgotten how to read the map.

Sources

← Back to the Desk — Technology