The PowerMac 7200, and possibly other machines of this date shipped with 
Open Firmware 1.0.5. This has the unfortunate property that, if set to 
boot from the internal HD, attempts to access it before it is on, gives up,
and halts. Result: your machine does not boot.

3 possible solutions: 
1) use 'BootX' to boot Linux indirectly through MacOS
2) Install Linux on an external HD and power up the external HD first,
3) Patch OpenFirmware to retry the disk until it finds it ready. 
   I shall cover my results with this more thouroughly under the 
   'Open Firmware' section.
-wdlvw@magma.ca