Se/30 Hard Drive Won't Mount

Is the SCSI bus properly terminated? If the drive is mounted internally you'll need to put a jumper across the termination pins on the drive (assuming of course the drive has termination resistors on it and doesn't need an external terminator.) In an external case it would likewise need some sort of termination.

try updating the drivers with hd sc setup.

... if we've got all our information straight.

Three out of three drives won't mount on the SE/30. They can be seen by disk utilities, but not mounted. Makes no difference whether the drive is mounted internally or externally.

The same drives will mount on another machine.

Terminators are used in internal and external configurations.

What SCSI ID is the drive set to use when connected to the SE/30? If that's the only SCSI device, you can use any ID but 7 (which is used by the system.) This is usually set with jumpers on a set of 6 pins, and is the arrangement particulars is usually printed on a label on the drive. If there is another device on the SCSI chain make sure that the two are not using the same ID.

Is there anything else that is used in common when you mount the drive internally and externally? Ribbon cable maybe? If not, it sounds like your SCSI bus might be hosed, which means hunting down a new logic board.

I used to think SCSI was the greatest thing on earth, but this reminds me of some of the nightmare situations I dealt with in SCSI. (shudder)

Loss of internal SCSI is not normally associated with capacitor leaks on the logic board (the most common sympton is loss of sound) but it is a possibility. Take out the logic board and examine it for leaks on capacitors. If any caps are leaking, clean the area around them with alcohol or de-ionised water. If you're confused, search out the information here or at 68KMLA about washing logic boards.

is it formatted as MacOS Extended? If so, i don't think the SE will "see" it, along with the Norton Floppy. If so, try reformatting it on the G3 with MacOS standard.

there is some dark colored residue on some of the circuitry.

That sounds as if it was an electrical burn. Most likely a component failure then burning out. I saw this type of thing several times in powersupplies.
In any event, now that we know that something has physically happened to the board, it's probably a good idea to replace it anyway. After all, once component failure happens, it's just a matter of time till the domino effect happens taking everything else out (kinda like how bad ram spreads to new areas)

I've had the same problem with my SE/30, a couple of workarounds were tried and eventually one of them was met with sucess. This problem was apparent when using non-standard half height 3.5" drives in the SE/30. It is well worth your efforts to get a larger HD to work in the machine, but it can try your patience. It was not a problem of termination, ribbon cable, or other faulty hardware. Some obscure problem exists during the final stage of formating, possibly the "Dirty ROM" issue affects the disk mounting phase.

1. Boot the machine from Disk Tools diskette with either an earlier or later OS than you are installing and try a low level format. I was sucessful when using a OS 7.6 Disk Tools diskette, but OS 7.5.3, OS 7.1, 7.0.1, or 6.0.8 would not mount the HD.

2. Try to format the HD in another system then place it back in the SE/30 to install the OS. (I would try this choice first).

3. Use a 3rd party disk utility like Silverlining for problem HD's.