How to fix an exFAT drive on a Mac

May 13, 2020

A couple of weeks ago, one of my wife's friend had an issue with an exFAT formatted external USB drive. exFAT, aka FAT64, is a Microsoft filesystem that comes in handy for drives you may want to be read and written both by PCs and Macs, in opposition to NTSF for which you need some third party software or some ability with the terminal to be able to write on.
This is the case with this friend too: she uses this drive to share data between her Mac and the PCs she finds at work, but this drive does often corrupt and when it does, the Mac cannot mount it. Usually, she goes back to work, mounts it on a PC and gets it fixed there, but now that she has been home for weeks, she did not know how to get it working again and, above all, retrieve the valuable data she had inside it. So she asked me.

Searching the web points to some code lines to be typed in the terminal to perform a fsck_exfat on the drive, a command line built into macOS since 2010, but something Disk Utility seems unable to do at first via its graphical interface.
Looking around a bit more, I found this thread suggesting one simpler solution: waiting.
Turns out, if you leave your drive plugged in for quite a while - the bigger the disk, the longer the time - it eventually gets mounted on the desktop. I forwarded this solution to this friend and, after a faithful wait, the drive was actually mounted and working.
Apparently, the problem seems to be related to not proper ejecting, especially if some files are still present in the trash upon disconnection. My guess here, is that macOS does silently perform fsch_exfat on its own while the drive is plugged if it senses something has gone bad with the volume, hence the need to leave it plugged in for long times, even if it looks as is the system is lying still. In fact, many report that the status light of their external HDs does actually blink while being connected to the Mac and not yet mounted on the desktop.
In case this simple solution does not work, looks like running the above-mentioned code in the Terminal does make the disk appear in Disk Utility and being fixed by it.

One final note: even if software tools are becoming increasingly powerful at fixing volumes' structures, regular backup is the only real guarantee your data will not be lost. If you can afford to have twice the HDs you own for data redundancy, go for it. If you can't, or want a remote copy of your data, among all the online backup solutions out there I highly suggest Backblaze. I subscribed to its unlimited plan 2 years ago and haven't worried about data loss since.

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