Learning & reference
A file system decides how bytes become named files — and every one has different rules for size, names, case and permissions. Those differences are where cross-platform bugs live. Here's what each one actually does, and the traps to test for.
Below the folder view, the file system is the bookkeeping layer between raw disk blocks and the files you see. Four jobs matter most for testing:
Maps human names to data, defines legal characters, case rules, max name/path length and how folders nest.
Size, timestamps, owner, and extras like NTFS alternate data streams or macOS resource forks / extended attributes.
Who can read/write/execute — POSIX rwx bits, or richer Access Control Lists (ACLs) on NTFS/APFS.
Journaling or copy-on-write so a crash mid-write doesn't corrupt the volume; checksums on modern systems.
The disk formats on laptops, servers, USB sticks and SD cards. Scroll sideways on a phone.
| File system | Native to | Max file size | Case | Permissions | Journaling | Watch out for |
|---|---|---|---|---|---|---|
| NTFSWindows | Windows (2000+) | ≈16 TB (up to 8 PB) | insensitive preserves case |
ACLs | Yes | Alternate data streams; reserved names (CON); 260-char legacy path cap |
| APFSmacOS / iOS | macOS 10.13+ (2017) | 8 EB | insensitive case-sensitive variant |
POSIX + ACLs | Copy-on-write | Stores names as Unicode NFD; snapshots & clones look like copies but share blocks |
| ext4Linux | Linux (default) | 16 TB | sensitive | POSIX | Yes | File.txt and file.txt coexist — breaks when synced to Windows/Mac |
| exFATremovable | USB / SD cards | 16 EB | insensitive | none | No | No permissions or journaling — fine for big files across OSes, risky for live data |
| FAT32universal | Everything (legacy) | 4 GB | insensitive | none | No | A single file can't reach 4 GB; volume capped ~2 TB — the classic "file too large" copy error |
| HFS+old macOS | macOS pre-2017 | 8 EB | insensitive HFSX is sensitive |
POSIX + ACLs | Yes | Superseded by APFS; still seen on old drives & Time Machine disks |
Windows' default since Windows 2000. Rich ACL permissions, journaling, compression, encryption (EFS) and alternate data streams (hidden data attached to a file).
QA trap: ADS means a file can carry hidden content your size/hash checks miss
(file.txt:secret). Downloads get a Zone.Identifier stream that triggers the
"file from the internet" block.
Default on modern Macs, iPhones and iPads. Copy-on-write, instant snapshots, space-sharing containers, and cheap file clones that share blocks until edited.
QA trap: stores filenames in Unicode NFD — a name typed as NFC on Windows won't byte-match, so "file not found" for a name you can clearly see.
The Linux workhorse. POSIX permissions, journaling, extents, and the fewest surprises — it stores names as raw bytes and is fully case-sensitive.
QA trap: case sensitivity is the big one — a repo with README.md and
Readme.md is valid on Linux but silently merges to one file on Windows/Mac.
The formats on USB drives and SD cards. No permissions, no journaling — maximum compatibility, minimum safety. exFAT lifts FAT32's limits.
QA trap: FAT32 rejects any single file ≥ 4 GB. A 4 GB video or backup that copies fine to NTFS/APFS fails on a FAT32 stick with a confusing error — always test this path.
People say "a POSIX file system", but POSIX is a specification: the rules an operating system follows for files, paths and permissions. It's why Linux and macOS feel alike — and why Windows doesn't.
POSIX (Portable Operating System Interface) standardises the file API so the same program works across
Unix-like systems. For files it pins down: a single /-rooted tree with / as
the only separator; case-sensitive byte-string names (anything except / and NUL);
the rwx owner/group/other permission model; and semantics like being able to delete or
rename a file that's still open.
Why it matters: Windows is not POSIX — it uses \, is
case-insensitive, blocks : * ? " < > | and reserved names, and locks open files.
Most "works on my Mac/Linux, breaks on Windows" bugs are really POSIX-vs-Windows differences.
See the filename compatibility matrix for the full list.
Beyond the local disk, files are also served over the network. These are protocols and devices, not disk formats — but they add their own rules on top of whatever file system sits underneath.
Windows-native file sharing —
mapped drives like \\server\share. Also spoken by macOS and by Linux via Samba. Carries
Windows-style rules (case-insensitive, reserved names) plus file locking.
Network File System — the Unix/Linux way to mount a remote directory as if it were local. POSIX semantics, case-sensitive, with uid/gid-based permissions that must map correctly between client and server.
Network-Attached Storage — an appliance full of disks that serves files to everyone on the network over SMB and/or NFS. It isn't a file system itself; internally it runs ext4, Btrfs or ZFS and exposes shares.
Storage-Area Network — serves raw block storage (like a bare disk) over a fast network. The client formats it with its own file system. Contrast with NAS, which serves ready-made files.
Not a file system —
a flat key→object store over HTTP. No real folders (just / in key names), no rename, no
partial in-place writes. This very site is served from it.
Apple Filing Protocol — the legacy Mac sharing protocol, now deprecated in favour of SMB. You'll still meet it on old NAS boxes and Time Machine setups.
Locking is advisory and protocol-specific, so two clients can clobber each other's writes. Latency turns fast local operations into timeouts and partial writes. Case & Unicode rules follow the underlying file system, so a Linux NFS export can hold names a Windows SMB client can't open. Permissions mean different things (NFS uid/gid vs SMB ACLs) and often need mapping.
QA trap: test the same upload/download flow against a network share, not just a local disk — path length, locking, interrupted transfers and permission mapping all behave differently.
Every quirk above has a matching test file — an empty file, a 4 GB+ file, case-colliding names, NFC vs NFD Unicode, reserved names and more.