On a Mac you need a card with PPC/OF ROM.
These are either cards by Sonnet, quite rare, or normal SIl 3112 cards flashed with the Wiebetech ROM.
I never bothered flashing, and my PMac loads the boot.img from a CF card on the IDE which than boots an RDB partioned SSD on the SIL ( x86 bios).
Do you need a PPC/OF ROM for a Pegasos?
Don't you have any issues when writing and deleting files using a CF card? I guess Morphos does not support Trim but a CF card is more a PATA drive than an SSD innit?