Regarding the $20K xray machine that only works in XP: It's entirely possible it actually does work with Vista or Win7 despite the manufacturer's disclaimer. Sometimes their disclaimer is just out of date (i.e. it was made several years ago), and sometimes it's just because they haven't adequately tested it on the newer OSes and don't want to commit to anything just yet. A lot of XP and Vista drivers just work on Win7. Another option that might work is use a newer OS, but run XP in a virtual machine (either using Win7's "virtual XP mode" or some other VM solution). If a piece of hardware really doesn't work on anything newer than XP, that really means that the associated device driver or the associated user-mode software doesn't work on anything newer than XP, and whose fault is that? The manufacturer. BTW can you provide a link to the manufacturer site where they state only XP is supported? I'm interested in following up with them as to why.
Your idea of moving the mass storage to another network-accessible machine is a good one. Clearly if this will be part of a $20K+ environment, the cost of this is trivial. The fact that there are viable workarounds supports my point that supporting > 2TB local volumes on XP is not an important issue. There are far more impactful issues for MS to be working on. Like oh, improving standards support in IE!!!