Can't Repair Disk Error Message After Using Passport on Second Computer

I have been using My Passport for Mac to back up an old MacBook Pro that runs on 10.10.2. I also was dragging songs from iTunes to the Passport for backing up. I recently bought a brand new MacBook Pro (running on Catalina) and used My Passport to move all my backed up mp3s to the new computer. That may have been a mistake - when I next connected My Passport to the old Mac I got the error message “OS X can’t repair the disk…You can still open or copy files on the disk. Back up the disk and reformat it as soon as possible.” I reformatted the Passport and backed up the old Mac again, and as an experiment, I then connected it to the new computer, moved a few mp3s onto it, and got the same error message when I reconnected the Passport to the old Mac.

I thought you could use the Passport on multiple computers but is the OS difference an issue?

Any advice or tips appreciated. Thank you.

Hi,

You may run First Aid on the drive: How to Check External Drive for Errors or Failure on macOS