I have done some additional testing.

It doesn't boot from an external hard drive as well with OS 7.5.3 for an SE/30.

I get the same errors: doesn't recognise the disk, unable to mount

I have checked the 3x Bourns filters: all measure 54 Ohms pin-to-pin in circuit so I guess these are OK.

I'm thinkig for either a broken trace from the SCSI chip somewhere or a faulty chip.

I inspected the leggs of the SCSI chip under a magnifying glass and they look good, no visible corrosion.

Does anyone have schematics or picture where all the pins of the SCSI chip need to go so I can test them for continuity ?

It would save me a lot of time as sorting them all out myself would take several hours.

I hope it's not the SCSI chip as I don't have the equipment to replace it.