corpnewt
commented
8 months ago USBMap is a tool intended for Hackintoshes - and while I imagine it could work on real macs given the proper preparations are taken, I do not have personal experience with any nuances that may be encountered. The first thing I'd do is verify the external HD works correctly on another machine. If it does - perhaps isntall an OS version that is supported by that iMac and verify again.
This may be a question better asked to those who maintain OCLP (as I'm assuming that's what you've used to install Monterey)?
-CorpNewt
Installed Monterey on 13.2 iMac yesterday w/16GB memory. External HDs (with Firewire 400, 800, & usb ports, connected to iMac with Firewire 800 via a thunderbolt to firewire adapter. The first day I was able to do Timemachine backup, however ever since every time I turn on the HD my mac freezes. Tried using the firewire 400 ports, and same problem. I have 2 OWC ext HDs and problem exists for both. My mac totally freezes. I have to force shutdown with rear button and restart, which is a very, very slow process. It may be the adapter, however it's shown as connected in the system report, and no real problems before installing Monterey. Any ideas on how to fix?