ExFAT drive won't mount

To repair a file system on Windows you need to use Disk Management. Within Disk Management you usually right click on the volume you want to repair.

For some reason Apple has not designed macOS to recognize all the variations of exFAT formatting regarding block sizes. If you want to use a drive with both macOS and Windows, then it is best to format (aka "erase") the drive using Disk Utility on macOS. Otherwise you need to specify a block size for exFAT that macOS understands if formatting the drive using Windows.

If you are only using this drive with macOS, then erase the drive as GUID partition and MacOS Extended (Journaled) for best results.

The difference in sizes may be due to macOS utilizing many hidden files for use with Finder or it may be due to the other file system using different block sizes.

insch wrote:

My understanding is that this is not a Windows issue but a Mac compatibility one? I’m assuming that Windows can’t change the allocation size but save the data on the drive.

It is most likely a compatibility issue since it mounts using Windows. The only way to have a compatible file system is to either use Windows to format exFAT using a compatible block/cluster size or the easier way is to use macOS Disk Utility to erase the drive. Of course you need to transfer the data from the drive before erasing it or the data will be destroyed as @Matti Haveri already mentioned here:

https://discussions.apple.com/thread/251748094?answerId=253365386022#253365386022

He mentioned a 128 KB cluster size, but in replying to another thread on these forums it seems that on more recent versions of macOS a 1024 KB cluster size may also be compatible (unfortunately Apple doesn't disclose this information AFAIK so it is hard to say which cluster sizes are compatible without experimenting).

If the drive was formatted using Windows, then the file system may contain a block size that macOS doesn't recognize. Backup your data using Windows, then use Disk Utility on the Mac to erase the physical drive as GUID partition and exFAT if you want to use the drive with both macOS and Windows. You may need to click on "View" within Disk Utility and select "Show All Devices" before the physical drive appears on the left pane of Disk Utility.

It seems 128 kB Allocation Unit Size should be used if exFAT is used on a Mac.

https://joshuatj.com/2015/05/13/how-to-format-your-hard-drive-hdd-for-mac-os-x-compatibility-with-the-correct-exfat-allocation-unit-size/

https://www.partitionwizard.com/partitionmagic/allocation-unit-size-exfat.html

Have you tried having Windows repair the exFAT volume?

Sometimes it just isn't possible to repair some file system issues. It is also possible there is a hardware issue with the external drive (the drive is failing, or there is an issue with the cable, adapter, etc. that has caused some type of file system corruption).

This is why people should always have frequent and regular backups of all their unique data whether it is on the main drive of the computer or whether it is stored on external media or locations.

So the drive works on Windows (a PC and also on Boot Camp), right?

If yes, then copy all files to another 128 kB Allocation Unit Size exFAT drive (I guess Boot Camp should be just the same but use a "real" PC if possible) and continue using it. If it works and after verifying all files are OK, use the other drive as a backup after reformatting also it. Or if the data is really very valuable get a 3rd drive as a backup and erase the original later when you are 100% sure you don't miss it.