Backup disk suddenly won't work Here's the error message I get: "An error occurred while copying files. The problem may be temporary. If the problem persists, use Disk Utility to repair your backup disk." When I try to follow that instruction, I'm informed that Disk Utility is unable to unmount the backup drive. This happens whether I have Time Machine turned on or off, and following ejecting and reconnecting the drive. Selecting Time Machine from System Preferences doesn't make any difference.

I'm running Yosemite 10.10.5 on a MacBook Pro 2.2 GHz Intel Core i7. My backup drive is a one-month-old My Passport for Mac, 1TB, which has been working perfectly up till now. Backups are using less than 1/10th of its capacity. It doesn't respond to my attempts to "get info," but I'm able to enter Time Machine OK.

Grateful for any help you can offer.
I've had this problem before. I think it is because the data is corrupt. What you need to do is restore the drive and create a new one. The easy way is to get a different computer and reformat it there, but if not, don't worry. Just go into Disk Utility and attempt to restore/reformat the drive. If it gives you an error like "Can't unmount the disk", follow this guide to forcefully unmounting a Time Machine backup disk. Then, you will have to setup a new backup.

Note: If your drive still gives problems after this, it may be broken and you may need to purchase a new one.