The indefatigable Murph Sewall passed on these bits of information about connecting SCSI devices when the power is on (don't do it!).
In response to my question about the risk of connecting or disconnecting a SCSI device without bothering to shut down, one person says they've done it from time to time and had a fuse blow once, possibly as a consequence. Another person said they disconnected an externally-terminated Ethernet SCSI device and had an immediate loss of hard drive response until shutting down (which took five minutes) and restarting. Four people were pretty sure it's a bad idea without really knowing why.
I did get one pretty good explanation which has convinced me to stop switching ADB devices around on live Macs as well. I asked Doug Larrick if I could pass what he told me along, and he said I could as long as everyone understands: "I speak not for my employer. All I have said here is from my own personal experience and knowledge of electrical and computer engineering (hey, that is what my degree is in :-) ); it is not the "official word" on anything."
Herewith, the "unofficial word" from Doug Larrick:
RS-232 [although of course, the Mac uses RS-422] has spoiled so many people it's amazing. That standard was designed specifically so that you could hook all the wires together at both ends and not hurt anything, with the consequence that you can connect and disconnect those cables at will with all the equipment powered on.
SCSI and the ADB are different beasts, however. Damage can occur when you make and break the connections. What typically happens is that the pins make contact at different times, and electricity flows the wrong way in some circuits on one end or the other, blowing them out. The worst-case scenario is that the reference power lines get connected last, so all the receivers get fried.
[Editor's note: If I remember correctly, frying the ADB, which some have done and others can't believe is possible, results in having to replace the motherboard unless you have a local wizard who can do component-level repair. I personally have plugged and unplugged ADB devices with the Mac on, but I don't generally recommend the practice. -Adam]
Now granted, there's some leeway here (it's heat that does the damage, and it takes a while for the heat to build up to the damage point), and I've been successful in occasionally transferring ADB devices around, but I wouldn't try it with SCSI - too many pins to try to connect all at once, too much potential for expensive damage. I did manage to fry a Sun keyboard by unplugging it and plugging it back in.
Dave Platt adds, "If you accidentally short the TERMPWR line to ground, when connecting or disconnecting a live SCSI cable, you will blow a fuse in each device which is providing terminator power. If this happens, external terminators will no longer function."