As far as I know, the SE/30 should boot fine with systems up to 7.5.5, no enablers needed -- assuming you have enough RAM, or course.
Can you check in another Mac whether the System Folder on your HD SC 20 is still blessed? I think some Macs which can't run 6.0.x (e.g. Quadra 700) are indeed able to install 6.0.8 on an external disk.
Does it boot successfully into 6.0.x? Maybe you're facing a SCSI problem and/or your floppy disk is misaligned.
Hope this helps, please let us know of your results for further directions.
Can you check in another Mac whether the System Folder on your HD SC 20 is still blessed? I think some Macs which can't run 6.0.x (e.g. Quadra 700) are indeed able to install 6.0.8 on an external disk.
Does it boot successfully into 6.0.x? Maybe you're facing a SCSI problem and/or your floppy disk is misaligned.
Hope this helps, please let us know of your results for further directions.