Entirely possible it is a parallel port CDROM drive, there were a few of those kind of things once upon a time.
If you plug a parallel port device into the A3000 SCSI connector (it's the same connector type for both ports) it usually shorts pin 25 to ground (5V termination power) which exceeds the series diode's foward current, causing it to go short circuit, causing a track on the bottom side of the PCB to fuse.
Afterwards there will be no termination power supplied by the Amiga to the SCSI bus and it'll refuse to boot in the same way as you've described.
Check this by measuring for 5V on pin 25 of the A3000 ext SCSI connector.
The diode is D800 and lives next to the SCSI port. The original is a 1N4148 or something, you can replace it with a standard 1N400? or any other (non-zener) diode you come across.
You'll need to take out the main board and repair the PCB track underneath, should be obvious where it's fused. You'll need to put a small wire from D800 to pin 25 of the connector.
Oh, and on some A3000 boards the silk screen detail for D800 was reversed. Obviously the cathode (stripe end) connects to pin 25 and the anode to +5V.
Double check the details at your own leisure, I could look it up in the service manual, but I'm too lazy to walk to the shelf to grab it. If you can't work it out, let me know and I'll get off my arse...
