WD My Book for Mac Backup Disk Not Available

When I try to initiate a back up from my iMac desktop running version 10.13.2 (High Sierra) to My Book for Mac (3T) external hard drive, I keep getting the message that the “Backup disk is not available”, even though the external hard drive is plugged directly into the computer. This used to run fine but over the last year or so it has become more difficult to make back ups. I don’t want to lose the data that is currently stored on the external hard drive. Hopefully, there is a way to work around having to restore the hard drive. Please help.

So how do you initiate a backup I mean are you trying to make Time Machine backup but unable to do so? Is your Macintosh HD APFS or HFS?

Yes, I’m trying to make a back up through Time Machine and I keep getting the message “Backup Disk is Not Available”. I don’t understand what you are asking concerning HD APFS or HFS. What do they mean?

APFS is the default file system of macOS High Sierra in case your start-up drive is SSD. Further, Time Machine doesn’t backup APFS. Pls read about your solution here https://support.apple.com/kb/PH25587?viewlocale=en_US&locale=en_US

I performed the steps in the link you mentioned and nothing got resolved. I still get the message that the backup disk is not available. Getting frustrated.

Did you look here? Not sure if it’s the same problem.