srieck,
If your SCSI hard drive is not showing up, it could be either drive stiction, improper SCSI termination, bad data cable, bad SCSI controller chip, System Folder isn't blessed or the jumpers on the drive are not set properly. Assuming the latter is properly set, let's start with the first possibility in the list.
Drive stiction happens when the layer of lubrication covering the Mylar platter of the disk has bonded with the read/write head, freezing it in its place. When you let your retro Mac sit for long periods of time unused or powered on, this bonding can occur. The easiest solution would be to take the drive out of the case, hold it in your hand (palm facing up) and quickly rotate horizontally (a quick flick of the wrist) the drive a couple times until free. Movement should be almost as if you were to spin a basketball on your index finger, with how you move the drive in your hand. Once done, place the drive back in place, reconnect the cables and power up your Mac. It should start booting.
If it still doesn't boot, try checking to see that the "termination enable" jumper is in place on the bottom of the drive. If the jumper is in place but still won't boot, try a different data cable. If still no luck, the System folder for the OS isn't "blessed" or may be corrupt.
To bless the System folder, simply insert the Disk Tools floppy of System 6 or 7 into the floppy drive and restart your Mac. Wait for it to load. If the icon for your hard drive shows up on the desktop, go ahead and double-click to open the drive up. You'll see the System folder in the list. Open up the System folder, then close it again. The System folder should now be blessed. Restart your Mac without the floppy and see if it boots off the hard drive.
If you're still not having any luck, then you'll need to double-check the jumper settings on the hard drive. Simply pull the drive back out and take notes of the make and model number of hard drive; you might need to Google it later. Chances are the company that created that drive is still in business or was acquired by one of the current popular brand companies. Seagate did swallow up Maxtor, Quantum and Conner brand hard drives, so go to Seagate for any old data sheets regarding your hard drive jumper settings. The same applies to other brands as well.
If you've double-checked the jumper settings but still can't get the drive to work, it could be the SCSI controller chip. On the SE/30, it is labeled UI12. It is a 53C80 square 44-pin PLCC chip. The symptom of a bad SCSI controller chip is when turning on the power of the SE/30, the monitor shows the bright rasters and an arrow pointer appears on the left upper corner. But the monitor doesn't show a smile Mac and the SE/30 doesn't start up. At this point, you'd just have to replace the chip; in my opinion, unless you know how to do so, don't even try.
Good luck and let us know what you find.
73s de Phreakout. :rambo: