https://en.wikipedia.org/wiki/ExFAT The error message, at the end, is a clue...the kernel doesn't understand
exfat. That might be because there's a kernel-related update in progress, waiting for a system reboot. I've seen the same message for
vfat after an upgrade. So reboot Mint and try mounting it again is the first step. If you continue to get the same message, try manually installing the packages
exfat-fuse and
exfat-utils as kernel-helpers.
To mend
exFAT you need a Windows machine that has exFAT drivers...anything from XP-SP3 onwards really. Linux can read it and write it, but it can't repair it. Leave that until last. There's a straightforward Linux emergency-recovery sequence for devices with no file system, or a wobbly file system, or even wobbly hardware like a faded-and-scratched DVD that just can't be replaced. You need a Linux machine with more than twice the size of the device as free space...more than 128GB free space in your case.
1) Install the
gddrescue package and use the
ddrescue command to clone the whole problem-device as a single file. The command looks something like...
ddrescue /dev/sdX image.img image.log
Now you have a backup in case it's an SD card hardware problem. If you like, you can use Mint's
USB Image Writer or the
Disks accessory to write the image.img file to a different removable store...it depends how important the missing files are to you, really. In any case, put the original SD card somewhere safe for Windows to look at later.
2) Install the
testdisk package and use the
photorec command...
photorec image.img
...to search the backup file for intact data files, even if there's bits of image missing or no file system at all.
That's everything Linux can do to preserve the original data. You can use a Windows machine to attempt a repair of the original SD. In my limited experience it usually decides the only hope is to reformat it, which doesn't help a lot.
Of Interest
You don't have to use exFAT and be tied to Microsoft OS. If you don't need files bigger than 4GiB the older FAT32 system is good for storage to at least 1TB.
As a user of 64GB media you might look at this...
viewtopic.php?f=50&p=1281718#p1281718 Not necessarily the problem you are having, but a real hazard in the market place.
