What version of Tableau Server? What command is being used to take the backup?
I saw the same error on 2019.1.3 when the default backup directory was set to the base drive in Windows Server 2012 and 2016 (ex: E:\). To verify this was the cause, I changed the path to a subdirectory:
tsm configuration set -k basefilepath.backuprestore -v "E:\TableauBackup"
tsm pending-changes apply
Verified the updated path:
tsm configuration get -k basefilepath.backuprestore
And then I re-ran the backup manually:
tsm maintenance backup -f <backup_file> -d
Backups seemed to work fine after that. I don't know if this applies to the original posters problem, but it may help someone else.