I have got the error:

exfat 1.3.0 ERROR: invalid VBR checksum

what fixed that was:

$ sudo exfatfsck /dev/sdc1
exfatfsck 1.3.0
Checking file system on /dev/sdc1.
ERROR: invalid VBR checksum 0xa36876fd (expected 0xa3716afc).
Fix (Y/N)? y
File system version 1.0
Sector size 512 bytes
Cluster size 32 KB
Volume size 29 GB
Used space 4012 KB
Available space 29 GB
Totally 0 directories and 0 files.
File system checking finished. ERRORS FOUND: 1, FIXED: 1.
(sdc1 being the partition that failed to mount)