Well it was indeed a faulty SCSI chip. I desoldered the old one with Chip Quick, put in a brand new "old stock" one I found on Ebay and the SE/30 immediately booted from it's HDD.

I left it running for 12 hours and ran hourly Norton Disk Doctor full surface tests with verify on the HDD and everything works fine.

Now FWB toolkit and Apple's HD SC Setup also recognize the HDD and were able to Initialize it, including successfull verification and mouting.

Did a clean install with OS 7.5.5 from CD-Rom: all well. Another one saved