Question for the community

I have a late-2015 iMac. 3.2ghz intel i5 cpu, 2gb AMD R9 gpu, 32gb ram. It currently has a split fusion drive: 1tb HDD / 24gb ssd Flash drive.

I want to upgrade the system to macOS Sequoia. My problem is the speed of using the internal drives.

So, my question is, I have a 500gb SSD external drive, I had macOS sequoia installed on it, but every time I restarted the iMac or turned it back on, I had to unplug and replug in the external drive to get it to work.

Is there something I'm doing wrong, or do I need a powered external drive for it to work properly?

-Thanks