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.