My time machine back up to my back up disk has been failing. I went to Apple forum and it suggested I partition they back up disk and start the back up over.
When I tried that it said the disk can't be "unmounted".
Hi, if the Disk is in use, it cannot be dismounted.
Turn off Time Machine & add that drive to Spotlight's Privacy pane.
Restart in safe Mode if needed, holding Shift key at bootup.
Retrieving data ...