The following command line, using tmutil, will force Time Machine to use your mounted disk as its destination. If the disk is mounted as /Volumes/MyBackupDisk, run:
sudo tmutil setdestination /Volumes/MyBackupDisk
use
sudo tmutil setdestination -a /Volumes/MyBackupDiskif you want to add to existing Time Machine drives
Use sudo tmutil setdestination -a /Volumes/MyBackupDisk if you want to add to existing Time Machine drives
P.S. tmutil is very useful, also for deleting backups and working over the "operation not permitted" error.
