Which disk format to use, and what each one does to your free space
APFS, Mac OS Extended, exFAT and FAT32 behave differently on space, compatibility and reliability. How to choose for each kind of drive.
The format of a drive decides more than compatibility. It changes how snapshots work, whether copying a file costs space, and whether a 5 GB video can be written at all.
The four you will meet
| Format | Use it for | Watch out for |
|---|---|---|
| APFS | Any drive used only with Macs | Not readable by Windows without software |
| Mac OS Extended | Older Macs, and Time Machine on some setups | Superseded by APFS for most uses |
| exFAT | Drives shared with Windows | Poor with Mac metadata, less resilient |
| FAT32 | Only when something specifically requires it | No file larger than 4 GB |
The FAT32 file size limit
This is the single most common surprise on this page. A FAT32 volume cannot store any individual file larger than 4 GB, so a video export or a disk image simply fails to copy with a message that sounds like a permissions problem. If you hit that, the format is the cause and reformatting as exFAT or APFS is the fix.
What APFS changes about space
- Clones. Copying a file within a volume costs almost nothing until one copy changes.
- Snapshots. Deleting files may not free space immediately, which is purgeable space.
- Shared containers. Several volumes draw from one pool, so free space is a property of the container.
- Space efficiency. Small files use less overhead than they did on the older format.
The first of those explains a common question: duplicating a large file in Finder appears to use no space, which is correct rather than a display error.
Choosing for a specific drive
- Only ever used with Macs: APFS.
- Time Machine destination: APFS on current macOS, which is what Time Machine will ask for.
- Shared with a Windows machine: exFAT, and expect Mac metadata to travel poorly.
- A camera, a car stereo or an old device: whatever it specifies, usually FAT32 or exFAT.
Reformatting
Changing format erases the drive, so the process is copy off, reformat in Disk Utility, copy back. On an external drive that is a slow afternoon rather than a risk, provided the copy is verified before the erase. Hidden folders travel too, which is why an external drive shows less free space than the files you copied.
Common questions
Should I format an external drive as APFS or exFAT?
APFS if the drive is only used with Macs, since it supports the metadata and features macOS relies on. exFAT only when you need to share it with Windows, accepting weaker metadata support.
Why can I not copy a large file to my external drive?
Almost certainly because the drive is formatted as FAT32, which cannot store any single file larger than 4 GB. Reformatting as exFAT or APFS removes the limit.
Does copying a file on APFS use disk space?
Not immediately. APFS clones the file so both copies share the same data until one changes, which is why duplicating a large file in Finder appears to cost nothing.
Is Mac OS Extended still worth using?
Rarely. APFS has replaced it for internal drives and most external use. Mac OS Extended remains relevant only for older macOS versions or specific backup setups that require it.