I just spent 5 minutes washing tar off my hands. Today I received a MacDirect SCSI enclosure with some Conner hard drive in it from eBay. It was described as working. I added it to my SCSI chain on my SE, made sure all the IDs/termination were right and whatnot, and Hard Disk Toolkit could not see it. I tried connecting it to the SE with no other external SCSI devices. Sad Mac with 0000000F 00000002. I tried every single ID with and without terminator all with the same result. I then opened the enclosure and was greeted by a bunch of tar surrounding each screw hole under the outer case. Before I knew it, my hands, my SyQuest, my CD-ROM drive, my pristine Extended II keyboard, the SE itself, and the ADB mouse II all had the tar on them. I took out the HD and foolishly forgot to take note of where the SCSI ID jumper cable things (from the ID selector on the outer case to some now unknown jumper on the HD). Since there is no documentation for these ancient hard drives, it seems that I dug myself into a hole. A hole filled with tar. ;D
A Filthy useless SCSI Enclosure covered in tar
The tar-like substance would have been some natural rubber isolating washers. The goo that you've discovered is the same stuff that is created inside printers, tape drives etc when the rollers disintegrate. You can clean up sometimes by sprinkling the goo with a fine powder (talcum powder, flour, custard powder etc) which will make the goo set sufficiently that it can be peeled off with a sharp knife.