Why do I get message Time Machine couldn't complete the backup to Segate Backup Plus Drive #1

[Merlady]

I have two Segate Backup Plus Slim for Mac 2TB External Hard Drive drives that I swap out weekly. I ejected #1 and connected #2 then tried to "Backup Now". I got the message that "Backup Disk Not Available. Try backup up again when Segate Backup Plus Drive #1 is available. The backup on #1 WAS completed. It was ejected; it didn't show on desktop or Preview. #2 was visible on desktop and listed in Preview. The drives are 3 years old and I've never had this problem. What's happening?

iMac 27" 2017
Running macOS Big Sur Ver. 11.6

 

[Merlady]

Disregard, I just solved my problem. Don't know how it got changed but I went into Time Machine preferences and had to check for it to backup to both drives. It's backing up to #2 now! Yea!

 

[Annie_M]

Disregard, I just solved my problem. Don't know how it got changed but I went into Time Machine preferences and had to check for it to backup to both drives. It's backing up to #2 now! Yea!

Thanks for the update… glad you got it working!