My Passport for Mac won't back up.

Which OS version are you now running? And, yes, that is now a different file system.

Is your backup drive still formatted Mac OS Extended?

If so, here is an article about Time Machine and the new filing system:

https://www.macworld.com/article/234913/macos-big-sur-supports-time-machine-on-apfs-formatted-drives-but-there-are-a-few-catches.html

You need to erase the hard drive.

YamahaC3 Said:

"My Passport for Mac won't back up. Notes: I have backed up this laptop with this same drive before, BUT, it now has a new system. Thought that was the problem. I erased the drive. Nope. Still failed. I ran disk utilities (First Aid). Still failed. Help! It's a 2T drive and I don't want to have to buy another one. Please help me get this one to work. Somebody. Anybody."

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Get this Serviced:

If the drive is failed, it is failed. You need a new hard drive. So, get this serviced.

Use Send-In Service:

Contact Apple Support:

• Contact Us - Choose Locations
• USA: 1(800)MY-APPLE
• Proceed from there as Necessary

Apple Send-In Service: At the moment, with the Coronavirus Pandemic, many Apple Stores are still closed at the moment. So, Send-In Service may be the only available option for getting this serviced. So, have Apple take this in through Send-In Service. They will send you a box; you would ship it out to Apple; their engineers would look at it; and the would send it back to you.

OR...

Take it to you local AASP (Apple Authorized Service Provider):

1. Go Here: Find Locations - Apple Authorized Reseller
2. Click: Service & Support
3. Enter: your location information
4. See: if there is an AASP nearby
5. Contact: an AASP that shows up, and find out more about the services that they offer to fix the device