Here is the other article from Low End Mac that I'm also getting information from. Key points are Bold, and especially key points are Blue, and Red means "WTF you've got to be kidding."
Low End Mac:
Mac USB & FireWire
Sathe Dilip - 1999.09.21
The following article is adapted from a posting to The iMac List written by Sathe Dilip. It is reprinted with his permission.
After ordering it in March, I finally received my Microtech XpressSCSI USB-to-SCSI converter cable about three weeks back. I wanted to use it with my SCSI Zip drive. I had avoided buying a USB Zip, thinking that I would be also able to use any other SCSI devices that are supported. The data sheet on USB XpressSCSI said it worked with a Zip, so I was counting on that.
After I received it, I called Microtech to ask them a couple of questions about connectors, etc. In that call, I was told that the Zip drive does not work alone with the XpressSCSI. You have to use a second SCSI device on the chain and terminate it, I was told. I have an old Quantum SCSI drive, which has a termination switch. So I connected everything as I was told and started everything, only to find that even the LED on the adapter won't light-up. Loaded the driver software - nothing.
I called Microtech and was told that XpressSCSI receives its power from the termpower (termination power) connection of the SCSI chain, and that I should look for the termpower jumper on the Quantum drive. I opened the drive, only to find that the drive did not have this jumper. (I exchanged some email in this regard with LaCie, but that's a different story). I was told that I will need to use a compatible device (listed on Microtech's web page) or a powered terminator (somewhere near $60).
I borrowed a Jaz drive (listed as compatible) from a friend to test the thing out and connected it at the end of the SCSI chain. This time the LED on XpressSCSI lit up, but I did not get any drive on the desktop. So I called Microtech again. This time the tech support guy was a good one. He explained what I needed to get the thing to work. The list included latest drivers for anything I connect on the SCSI chain and the latest update for USB class devices from Apple. Microtech's manual says you will be prompted (by their software?) if the device you have connected needs a driver, but I never received a prompt.
In the meantime, I had also researched the subject of termpower and found that:
It is just a 5 volt supply (actually they use a Schottky diode in series with the 5 volts from the drive's power supply, so it is a bit less, something like 4.8 volts)
Apple's implementation of SCSI with the 25 pin connector is not per the SCSI standard.
Older Macs and some Apple SCSI devices did not use the termpower connection over the cable.
The newer Macs connect pin number 25 of the SCSI connector to enable use of termpower.
My Quantum drive does not support term power.
Armed with this knowledge, I decided to make the Zip and Quantum drives work with the SCSI adapter. I downloaded the latest drivers for the Zip, as well as Apple's latest update for USB class devices. I downloaded the upgrade to Silverlining (drivers used by LaCie, makers of Quantum drives). I installed these on the iMac and the drive, respectively. Then I proceeded with creating a 25 pin adapter that had pin no. 25 fed with an external 5 volt supply (pin 24 is ground). I connected this at the end of the SCSI chain, over the second 25 pin connector on the Quantum.
Now I connected everything - and sure enough, everything worked. I had the Quantum and the Zip showing on my desktop. I observed that Zip Tools from Iomega did not work properly with my drive, but that is probably not a Microtech problem. (I can use the SCSI Zip drive alone with the iMac when I use the 5 volt adapter I made.)
In short, the USB XpressSCSI works, but it needs certain considerations (compatible devices, termpower, latest drivers). Microtech manuals are not much help. Their tech support gives out advice in bits and pieces. If you do not have SCSI devices from your earlier Macs and you just need to connect a storage device to your iMac, I say just buy a USB ready device.
As to why Microtech chose to get the device power from termpower and not from the USB port (all active ports are supposed to supply up to 500 mA; this adapter needs about 250 mA), I don't understand. A similar device manufactured by Newer Tech seems to have the same approach to device power, so it may have something to do with the SCSI standard.
I have been wondering why the manufacturers have chosen SCSI termpower as the power source instead of taking the power from the USB port. I am coming to believe that they were probably targeting notebook users in doing so. They did not want to deplete the notebook batteries faster by taking the power from the USB port. However, they could have added a switch and left the choice to the end user.