2017 iMac Sequoia not showing up in boot mode

Hey team,

Used OCLP to successfully install 15.6 Sequoia onto an external SSD (Crucial P310 2TB M.2 2280 NVMe) using my M1 MBP as a conduit.

I have been running Ventura on another external SSD on my iMac using the same method for a couple years just fine, but the SSD is too small and the internal drives are pure HDD (not fusion) and painfully slow, hence wanting the upgrade.

I attempted to just run OCLP on the iMac directly but during installation restarts, the iMac boots to the internal drive automatically - so during install of a macOS it would always be interrupted.

When I boot my MBP to view startup options, it shows the sequoia SSD and boots just fine.

When I do the same on the iMac, it only shows internal drive or EFI boot.

The drive shows up when I go into system settings and change Startup disk, but upon restart it will just go straight to the internal drive.

Not sure what I’m doing wrong at this point. I understand it’s not compatible to install without OCLP but I’ve seen plenty of successful posts here.

I’ve wiped and repeated the install a couple of times in case there was an issue or corrupted install, including simply downloading the macOS straight from the MBP App Store and installing onto the drive. All with the same results.

Would appreciate any feedback that comes to mind!