"Incompatible Disk" message

[davidlv]

Have an old MBP had Big Sur installed with OCLP, need High Sierra installed for access to old 32bit software.
Made a new volume on the 256GB SSD and installed High Sierra successfully. On reboot, got that incompatible disk message. Dismissed that message and everything seems to work OK.
Tried booting from the HS USB installer, and erased the Big Sur volume, also deleted the content of the EFI partition after mounting it with the MountEFI utility.
Still got that same message. So booted from the HS USB, nuked the SSD and reinstalled HS.
But that same message appears! What could be the cause of that message?

 

[avz]

Reset NVRAM?

 

[Fishrrman]

I believe you're getting the "incompatible disk" alert because you have Big Sur on the same drive and it uses a different disk volume configuration -- something that is not "recognized" by High Sierra (which is of course older).

My advice is to just ignore the alert, and realize that there are some things (on the Big Sur side) that you won't be able to access while using High Sierra.

You'd get the same alert even if you had installed HS on an external drive (which is what I would have recommended).

 

[davidlv]

I believe you're getting the "incompatible disk" alert because you have Big Sur on the same drive and it uses a different disk volume configuration -- something that is not "recognized" by High Sierra (which is of course older).

My advice is to just ignore the alert, and realize that there are some things (on the Big Sur side) that you won't be able to access while using High Sierra.

You'd get the same alert even if you had installed HS on an external drive (which is what I would have recommended).

Thanks for the advice. Indeed, HS seems to run fine after dismissing that alert. I have also heard that Mojave will also allow 32-bit apps to run, which is the whole and sole reason for installing the older Os on this machine, which will be kept offline and used to access old mail using an older 32-bit mail client that is now abandoned.
Much obliged for the reply.