I have an Macmini2,1 from mid 2007 with similar specs [1] and presumably
similar firmware. I tested a whole lot of combinations for booting it
and
came to the following conclusion:
To boot OpenBSD you have to use the internal SATA or ATA devices.
That means you need to use the CD/DVD-drive as installation medium
(installXX.iso). Alternatively you can replace the CD/DVD-drive with a
hard drive and use that as installation medium.
Booting from USB only works for MacOS X.
EFI-Booting (32-bit on my setup) works fine. To speed up boot time after
the installation of OpenBSD, use the Mac-proprietary bless(1) command
[2]
once (by booting some MacOS or MacOS installation medium). It writes
your preferred boot volume into NV-RAM, thus skips the boot volume
search (grey boot screen after turning the machine on) and continues
booting OpenBSD immediately.
--multifred
[1]:
https://everymac.com/systems/apple/mac_mini/specs/mac-mini-core-2-duo-2.0-specs.html
[2]: https://ss64.com/osx/bless.html
