We have a precision 420 with internal Plexwriter 12/4/32 connected to the internal scsi bus. When we connect the HP6350 scanner to the external scsi port the Plexwriter will not work properly. The scanner works fine.
There are a number of possibilities that could be hosing up your system.
Make sure there aren't any ID conflicts. It's a common simple mistake and could produce the symptoms you describe.
Another thing, and something I'm leaning towards as your problem, is a termination problem. Termination on the host adapter is (more often than not) enabled by default. With the Plexwriter's termination and card's termination enabled, things should be fine. However, when you attach the scanner, it's onboard termination would over-terminate the scsi bus, in which case, who knows what can happen. In your case, the Plexwriter hoses up. To fix this, you can dissable the termination on the host adapter FOR THAT PARTICULAR CHANNEL ONLY. That would leave the ends term-ed as long as the scanner remains attached.
I checked and the aren't any ID conflicts on the buss. I disabled the termination on the host adapter using the Adaptect SCSISelect Utility. Didn't help. The scanner still works but I can't write data to the Plextor CD-RW. When the computer boots the adaptec driver for the AIC 7899 host adapter starts and shows the connection rate for each device. The Plextor CD-RW and the JAZ drive are 40MBS but the HP scanner is "asyn". Could this be causing a problem on the buss?
If your scanner is initiating asych transfers, then it's possible that could be slowing the data transfers to the cd-rw, causing it to timeout. However, I'm a bit rusty and don't remember if it's ID specific or whether it affects all devices on the bus.
Are you using all three connectors (2 internal and 1 external) on the host adapter board? If so, this could also be your problem. You can only use 2 of the 3 connectors at one time, on a single channel card. However, you state that your BIOS is reporting you have an AIC 7899, which is a dual cannel chip, so you should be fine if you are keeping the devices separated that way.
I've got a ws420, internal plextor drives (SCSI) and I tried to connect a HP6350 on the external scsi port. Under Windows 2000 it was connected through USB and now I'm running windows NT 4.0 (SP6a). It seems that there is a software bug in most probably the HP drivers, Everything went well untill I installed the scanner. I got a nice blue screen. Right now I'm looking for the answer to fix this problem, or a way to connect USB under windows NT. There are no ID conflicts, and the scanner reports that it is asynch, but my plextor burner works really fine. So I think the scanner is the problem here.