As often happens when I post a problem here, I start to do more research and usually find the solution to my problem. Such is the case with this problem.

As I thought about the issue more, I went to the server and ran Diskpart locally on the server. I checked the attributes for the drive where the ProSeries Workstation files are located. It said the drive was not write protected, but I ran the clear readonly attributes anyway, and voila! Now the workstation install works!

I don't understand why the write protection was applied on this server drive only. I bet Microsuck doesn't know either, but at least I can run my Proseries programs now.