Mac (Intel) won't boot from clone

[prouss]

I don't know if this is a Mac issue, a SD issue or a Seagate issue, but after a couple of years of working fine - backing up and booting from - my old Seagate external failed, and since it was still under warranty they sent me a replacement. This one works fine for backing up (in the sense that the backups complete and the date seem to be there), but it does not show up as an available hard drive to boot from on startup. (I'm using an old Intel Macbook Air running Big Sur.)

Seagate is giving me a runaround, telling me that if it's backing up and is visible in Finder as an available drive then it's working perfectly. They insist it's an Apple issue and there's some "operating system" software I need to download from Apple and install on the drive. (Mind you, they didn't seem to understand the concept of "booting from an external drive", which is kind of mind-boggling for a drive company.) I never had to do anything like with previous Seagate drives or drives from other companies - just plug and play (assuming formatted properly - this is APFS).

Any suggestions? Thanks

[dnanian]

Seems likely you didn't make a full "Erase, then copy" Backup. Please see this post:

https://www.shirtpocket.com/blog/ind...ot_a_marathon/