I had a lot of problems with my PC scsi card, also an Adaptec (probably the same one: in your device manager does it say AIC-7850 ?)
I had a 1gb HD in my A4000T and an external CD-rom, and a Zip-100 external, all of them SCSI. They all worked fine on the amiga. When I connected the ZIp-100 to the PC externally, it worked fine. As soon as I added the internal HD, no SCSI devices showed. It was like a SCSI conflict with the IDs, but they were all different. Then I unpluggedd the Zip drive and the internal 1gb drive was visible.
I tried quite a few combinations and my conclusion was that I couldn't have an internal and external device connected to that Adaptec card at the same time.
So I asked Adaptec tech suport for help. And I am sorry to report that they bounced me around back and forth via email, asking really stupid stuff IN EVERY email, about termination, SCSI IDs and the possibility that the devices were damaged. No matter how many times I told them the IDs and termination were not the problem, they just didn't want to know. In the end they routed me over to a survey screen that asks how well the tech support helped me. My comments were not favourable.
So, at the moment I have an external 1 gb HD that is connected to the A4000T and the PC simultaneously (but not when they are both on, as it corrupts the Amiga side something terrible). I use it to transfer files. But the Adaptec won't take an internal device too. I think you have a similar problem to me, pal.