@Effy
Hold the 'esc' key during startup, and you will be in the BlizzardPPC boards menu.
There is a setting to disable IDE (scsi.device) someplace in there, called 'SCSIpatch' IIRC in the 'system' menu. (In order to shorten the boot time when no IDE drives are connected).
@FrAmiga
You can create an 'IDE terminator' with a IDE cable and two resistors. Have a look on aminet, search for 'IDE terminator' or 'IDE killer'
This stops the Amiga for searching for drives on the onboard IDE.
-Paul