My Time machine Back up has stopped working with the error message "Unable to complete backup. An error occurred while creating the backup folder."

I ran into the same problem on my WD My Passport backup drive, and came here looking for the solution. I think I have it figured out.

I'm using the hardware encryption on the drive, which requires a password in the WD Unlocker at the time the drive is mounted. I believe that the problem is that the Mac's energy saving rules are allowing that drive to be powered down after a period of inactivity, after which the OS still thinks it's mounted, but the drive encryption has re-locked. I figured this out by trying to browse the drive after I noticed one of these failed backups; I was able to see the first couple of levels of folders, but when I went any deeper they came up empty, and those I/O errors showed up in the console logs again. I logged back into WD Unlocker and the errors stopped.

I fixed it (I hope... I'll know for sure in the morning) by disabling the "Put hard disks to sleep when possible" checkbox in the Energy Saver preferences for Power Adapter.

Followup to my last message. That Energy Saver change was not enough... the next automatic backup also failed, with a different error:

12/7/15 4:51:43.682 PM com.apple.backupd[5256]: Error: (22) setxattr for key:com.apple.backupd.HostUUID path:/Volumes/My Passport/Backups.backupdb/mymac 3 size:37

I should note that these problems started after I migrated to a new MBP running El Capitan a couple of days ago; the old system was on Mavericks (using the same backup drive) and never had this issue.

TIME MACHINE PROBLEM: "Unable to complete backup. An error occurred while creating the backup folder."

MY COMPUTER: OS X Yosemite, Version 10.10.5 (14F1509)

iMac (27-inch, Late 2012), Processor 2.9 GHz Intel Core 15, Memory 8 GB 1600 MHz DDR3,

Startup Disk Macintosh HD

Graphics NVIDIA GeForce GTX 660M 512 MB

WHAT I'M TRYING TO BACK UP & WHERE:

Macintosh HD, on (new) WD MyBook 3TB external disk via usb 3.0

PROBLEM:

Initial backup succeeded, as did subsequent hourly backups, until 1:06 pm today.

Then this error message started to pop up: "Time Machine could not complete the backup.

"Unable to complete backup. An error occurred while creating the backup folder."

CONSOLE info on one of the last failed backups:

12/13/15 2:36:41.683 PM com.apple.backupd[2062]: Backing up to /dev/disk2s2: /Volumes/My Book for Mac/Backups.backupdb

12/13/15 2:36:42.401 PM com.apple.backupd[2062]: Error: (22) setxattr for key:com.apple.backupd.HostUUID path:/Volumes/My Book for Mac/Backups.backupdb/Colene’s iMac size:37

12/13/15 2:36:42.402 PM com.apple.backupd[2062]: Backup failed with error 2: 2

Can you help me get this working again?

Col/Abe