I have Windows 2012 Server with 2 ext. USB HDs. During the server backup job creation process, I kept both HDs connected. It labeled them "...DIsk1 " and " Disk2". So I thought I could keep one off-site and rotate them weekly. But it fails to backup whenever I switch them out and each week I end up creating new backup job to get the backup going. I thought if I connect both HDs to the server during the job setup process, Windows will remember both and use either one when the job is to run.
Then disconnect this drive and create a job to take backups to this HdD2 on Every second day. This should work when the right drive is there.
Or you get a copy of Paragon Hard Disk Manager 15 Server Basic and plan a backup job to external drive Emfor example. And it will take backups not depending on which drive is connected. Exactly what you wanted from Win Server Backup.