This solution worked for me, after searching and trying all possible solutions on multiple forums. Some context: Running MacOS Big Sur 11.6, Disk utility first aid said "seems to be ok", fsck_exfat returned "seems to be ok", disk was showing up on Disk utility but could not be mounted.
Run "sudo /sbin/mount_exfat /dev/disk2s2 /Volumes/External-HDD/" on the terminal. Enter account password when prompted. My disk was formatted as exfat (use mount_<your disk format type>) and diskutil list showed the disk identifier as disk2s2. You have to first mkdir /Volumes/External-HDD/. Hope this helps.
BinditaC
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