If parts of the backup database have been on the failed drive and you have not created a backup of this, it helps only to delete the entire backup database and start over backups from the scratch.
On the server open a command prompt.
Enter following commands:
net stop pdl
net stop whsbackup
d:
cd \folders\{00008086-058D-4C89-AB57-A7F909A47AB4}
del *.*
(You can also perform the deletion of all files in that folder from Windows Explorer after stopping the mentioned services, if you are not so sure about the command prompt.)
After completing the deletion (ensure that no files did remain in the folder) restart either the services or the server.
Btw. - external disks as part of the normal storage pool are not recommended, since people had often problems in the past (depending from the hardware - enclosure, cabling, USB port).
Best greetings from Germany
Olaf
