Okay after some searching I found this thread: USB to SATA adaptors and SWAP space nightmare - #19 by cgfrost
The problem is the drives aren’t fully wiped when doing an install, even though the installer says it will.
I ran blkdiscard /dev/<drive> on both my mirrored drives, and on reinstall it works fine.
Hence why your switch worked, because the new drives didn’t have TrueNAS already installed.