iMac won't boot from external SSD after Big Sur installation

[Anchorman10]

After the Fusion Drive in my iMac (Late 2014) began to slow down, I installed Catalina on an external SSD which booted flawlessly until Big Sur.

Repeated attempts to install Big Sur on the external SSD (which was erased) resulted in error messages. I called Apple Support and was told that installing on an external SSD wasn't a path recognized by Big Sur. (I also can't reinstall Catalina because Big Sur won't open it.)

I had no choice but to install Big Sur on the Fusion Drive which is painfully slow. I've been searching for a work around, but haven't found one yet.

 

[sirozha]

You should find out why your Fusion Drive is painfully slow. It shouldn’t be. The Fusion Drive in my 2012 Mac Mini is as fast as it has ever been.

I also have a Fusion Drive in my late 2014 iMac, but I replaced the 1TB HD with a 2TB SSD about 6 months ago. So, it’s a Fusion Drive that consists of two SSDs. I know that a lot of people think it makes no sense, but it works really well, so it added 128 GB of the original SSD that came with the original Fusion Drive to the newly installed 2 TB SSD in one volume.

My question is about the process of installing Big Sur onto a Fusion Drive on a 2014 5K iMac. Did you have any issues?