Stop iMac trying to access dead fusion HDD

I have a 2017 iMac and a few months ago the fusion drive started failing (intermittent access issues). No problem, I now have a 2TB SSD in an external thunderbolt enclosure acting as the boot drive. Works great. (I couldn't be bothered stuffing about opening the iMac, far too much effort to work on and re-seal) I split the old fusion drive so the HDD and 120gb SSD are showing as separate drives (both re-formatted and empty), so I can use the SSD part if I need to (probably never will though). The mac has recently started making noises like it's trying to access the HDD. I can hear it trying to spin up, an audible (not loud, but loud enough to be really annoying) "ka-chunk...ka-chunk...ka-chunk.....................ka-chunk............ka-chunk...ka-chunk...ka-chunk.............ka-chunk". kind of randomly timed. Is there anyway to tell the iMac to stop using/trying access the drive? A logical/software equivalent to removing it? There's no 'Eject' option available in Disk Utility, and unmounting makes no difference. iMac, 3.8GHz i5, 2017 running Ventura 13.7