Back up to external hard drive with Time Machine

zoeybird wrote:

I apologize; I wasn’t terribly clear. the Mac is backing up to the second external hard drive, but the first hard drive is not backing up.

If the drive is not backing up it is one indicator of a failing drive.

Repair a storage device in Disk Utility on Mac

https://support.apple.com/en-in/guide/disk-utility/repair-a-disk-dskutl1040/mac

zoeybird wrote:

Hello!

I have a conundrum. I'm trying to help a friend (who is on a Mac) back up his Mac and one external hard drive to a second external hard drive. I thought we had it set so that the second external hard drive was set to be the backup disk in Time Machine. We also made sure the first external hard drive was included in the backup through Time Machine preferences. However, that's not working.

Any ideas? Thank you for your advice!

If you hold the Option key and click the TM icon in the upper right menu bar> Browse Other Backup Disk