Why do files copy again to an exFAT or FAT32 drive on a Mac?

By Kayabyte · Last updated 13 November 2026

Short answer: Sync tools decide a file has changed by comparing its size and modification time. FAT32 stores times with a 2-second resolution, so a file copied there can come back a moment different from the original, and tools that compare exactly see a change every run. A second cause is that some formats store local time rather than UTC, so a time zone or daylight saving change can shift every file by an hour. The fix is a tolerance. With rsync use --modify-window=2. Pairfold applies a tolerance for FAT-type drives and, in our time-zone tests, made no changes after a zone change.

The 2-second rule

FAT file systems record times in 2-second steps. If the original has 10:00:01, the copy can carry 10:00:00. Compared exactly, the two never match. exFAT keeps finer times but is still a format with its own quirks, so a drive from a camera or Windows PC may behave differently from an APFS one. Check Microsoft’s documentation if you need the exact rules for your format.

Time zones and daylight saving

When a format stores local time, the stored value does not change when you move zones, but the way a Mac reads it can. After a time zone or clock change a naive comparison sees every file as an hour (or more) different. If everything copies again right after a clock change, this is why.

Other reasons a sync copies everything again

The drive was not mounted and the tool copied into a folder on the Mac’s own disk named like the drive.
A name differs: Unicode accents can be stored in two forms that look the same.
FAT32 cannot hold a file over 4 GB, so a big file fails every time.
Names with characters FAT does not allow, such as a colon or question mark, are refused.

Fix with rsync

rsync -avn --modify-window=2 ~/Folder/ /Volumes/Drive/Folder/ treats times up to 2 seconds apart as equal. For a time zone shift of whole hours you may need --size-only or -c (checksum), which are slower or less strict. Run with -n first.

With Pairfold

Pairfold compares size and modification time with a tolerance for FAT, and can verify each copy with a checksum. In our tests on disk images in a macOS 26 virtual machine, changing the Mac’s time zone (GMT to New York with exFAT, GMT to Tokyo with FAT32) was followed by a run with zero changes. On FAT32, items over 4 GB and names the disk cannot store are listed as left alone, with the reason, instead of failing.

Limits: we tested disk images, not every camera card or old drive. Pairfold keeps file dates when it copies; the exact result depends on what the file system can store.

Steps with Pairfold

Create a pair with New Pair, with a folder on the exFAT or FAT32 drive.
Click Check and Preview. Items it cannot put on the drive appear as left alone, with the reason.
Click Sync, then run Check and Preview again: it should list nothing to do.

Get Pairfold: see the app page for where to get it

Questions

Why does my Mac sync copy the same files every time to a USB stick? Most often the 2-second time resolution of FAT, a changed time zone, or the stick not being mounted at the path you expect.

Should I format the drive as APFS? For a drive only used with Macs, APFS or Mac OS Extended avoids these problems. exFAT is for sharing with Windows.

Does Pairfold copy files larger than 4 GB to FAT32? No: FAT32 cannot hold them. They are listed as left alone.

Apple, Mac, macOS, Finder, Time Machine and iCloud are trademarks of Apple Inc. Pairfold is made by Kayabyte and is not affiliated with or endorsed by Apple.