Boot 2017 5K iMac from Firewire

[imrazor]

My iMac has a very small internal SSD. I tried to use a Crucial X9 Pro 1TB as a boot drive, but half the time it would hang at boot. I then installed Ventura onto a 2TB SATA SSD in a G-drive mini enclosure. This boots quite reliably. However I occasionally get spinning beach balls, possibly indicating that the I/O from the USB enclosure is overwhelming the poor old i5 in this Mac.

So as an experiment I’d like to try booting off of a FireWire 800 enclosure to reduce the load on the CPU. However Ventura actively prohibits this. After moving the SSD from the G-Drive enclosure into a FireWire enclosure it refused to boot even though it was visible from the desktop. And trying to reinstall Ventura to the FireWire drive from Recovery mode says that it can’t install to a FireWire device.

It appears that OpenCore supports booting Ventura from FireWire. That said, if possible I’d like to avoid reinstalling the OS. Do alternative bootloaders like Refind also support FireWire booting?

 

[DeltaMac]

What adapter are you using to connect your Firewire drive to Thunderbolt 3?

 

[imrazor]

Apple TB3->TB2 adapter, then connected to an Apple TB2->FW800 adapter. The enclosure has its own power supply.

 

[Fishrrman]

You DON'T want to boot from Firewire.
Way WAY too slow.

And, as you have already discovered:
"And trying to reinstall Ventura to the FireWire drive from Recovery mode says that it can’t install to a FireWire device."

That you are having problems booting from your existing USB drives suggests that there's something "in your software mix" (or possibly even external hardware) that is causing problems.

That's where I'd be looking.

Have you tried a safe boot (hold down shift key continuously at boot)?