A file system is the bookkeeping layer on a partition: how files are named, where their contents live, what metadata is recorded. For a drive that stays inside one computer, the choice is usually made for you. For a drive you carry between computers, it is a decision worth five minutes of thought.
The three you will actually meet
FAT32
Old, simple and readable by essentially everything with a USB port — including firmware, which is the reason it still matters. Its defining limitation is a maximum file size of 4 GB minus one byte. Since most current desktop operating system images are larger than that, FAT32 alone is no longer sufficient for a drive that carries them.
exFAT
Designed as FAT32 without the size ceilings, and now supported natively on Windows, macOS and current Linux kernels. Files can be enormous. It records less metadata than a full-featured file system and keeps no journal, which makes it light but also means an interrupted write is more likely to leave a mess.
NTFS
The full-featured Windows file system: journalled, permission-aware, compression-capable, no practical file size limit. Reading it on Linux is routine; writing is well supported by current drivers. On macOS, write support requires additional software, which is the main reason people pass it over for portable media.
At a glance
| FAT32 | exFAT | NTFS | |
|---|---|---|---|
| Maximum file size | Just under 4 GB | Effectively unlimited | Effectively unlimited |
| Windows | Read and write | Read and write | Read and write |
| macOS | Read and write | Read and write | Read only by default |
| Linux | Read and write | Read and write | Read and write |
| Firmware readability | Very wide | Varies by vendor | Varies by vendor |
| Journal | No | No | Yes |
The two-partition arrangement
Multiboot loaders sidestep the trade-off by splitting the drive. A small partition formatted so firmware can definitely read it holds the loader and its EFI files. A large data partition, formatted for capacity and convenience, holds your images. Firmware only needs to understand the small one; your desktop only cares about the large one.
If the drive must be writable on macOS as well as Windows, exFAT is the pragmatic answer. If it lives in a Windows and Linux world, NTFS gives you journalling for the same file-size freedom. FAT32 is now only the right answer for small drives holding small files.
Allocation unit size
Formatting tools offer a cluster or allocation unit size. Large clusters waste space on small files but reduce bookkeeping overhead on large ones. A drive whose contents are a dozen multi-gigabyte images is exactly the case where a larger cluster size is sensible; the default your formatting tool picks for the drive's capacity is almost always fine, and this is not a setting worth agonising over.
Habits that keep a portable drive healthy
- Eject rather than unplug. On file systems without a journal this is not a formality — it is the mechanism that ensures pending writes have actually landed.
- Do not fill the drive completely. Leaving headroom reduces fragmentation and gives the controller room to work.
- Keep a checksum list. A small text file at the root recording the published hash of each image lets you re-verify later without hunting for the source page.
- Copy large files in one operation. Interrupted transfers to a non-journalled file system are the most common source of subtly corrupt images.
What about the Linux-native file systems
Ext4, XFS and Btrfs are all excellent, and some multiboot loaders support them on the data partition. They bring journalling, checksums and good behaviour under load. The cost is that Windows and macOS cannot read them without extra software, so they suit a drive that lives in a Linux-only workflow and suit nothing else. If your drive's whole purpose is to move between unfamiliar machines, stay with something those machines already understand.