Jerome Fine replies: NOTE: My experience is ONLY with Qbus PDP-11 Systems, so if my comments do not apply, please disregard! About ten years ago, I acquired a number of 5 1/4" ESDI disc drives of 600 MBytes which were being used in groups of 4 as an emulation of some kind. Each box had its own 300 Watt PC power supply with 5v/12v along with 2 fans for internal cooling. I also acquired a number of Sigma RQD11-EC ESDI Qbus disk controllers. I removed the raw ESDI disk drives, the PC power supply and the fans from each box and ran the drives as they were without an enclosure just sitting on top of a cardboard carton to ensure both electrical isolation and perhaps a bit of vibration isolation. I ALSO placed a fan beside EACH ESDI disk drive for cooling. As an additional precaution, I also attached a ground wire to the frame of the Qbus (BA-123 in my case) and the PC power supply. Since the fastest backup is disk drive to disk drive, I suggest that you use an extra drive just for backup if you have enough ESDI disk drives - if you have a backup requirement. I suggest you take Don's advice and power the ESDI disk drives from a separate PC power supply. Since the BA11-K has no drive bays, this seems like a good solution in any case. Unless many non-computer people are using the system, there is no need to enclose the disk drives. As I mentioned above, assuming that the XT-4170 generates enough heat, it is probably best to use a fan for cooling! ALSO - for anyone doing this with a Qbus system and using TK50 or TK70 tape drives, the use of the external PC power supply was NEVER successful. Never found out why, just always had to used to internal power form the BA-23 or the BA-123. As for slowly cooking the power supply for the BA-123, I found out the hard way after about 2 years when I was still using RD53 hard drives. Sincerely yours, Jerome Fine -- If you attempted to send a reply and the original e-mail address has been discontinued due a high volume of junk e-mail, then the semi-permanent e-mail address can be obtained by replacing the four characters preceding the 'at' with the four digits of the current year.