So naturally I started at the other end and moved the 120gb drive to my PC and cloned everything without any problems. Took a while, but worked pretty well.
Then I tried to put the 750gb drive into the network housing and the 320gb drive into my USB box, and sort out that transfer. And got nowhere. It recognised the drive, but couldn't format it or access it at all.
So, I decided to try formatting the 750gb drive as FAT32 first - PCs can't do this on drives bigger than 32gb, but Macs can, so I put the drive into the USB housing and used my iBook to reformat it. Took hours, but I assumed that it would fix things.
Unfortunately the result turned out to be another impasse - the drive housing again knew that the drive was there, but couldn't write to it or access it.
So I set out to contact the vendor and the manufacturer yesterday, and discovered in passing that while there is nothing about maximum drive size on the manufacturer's web site for that specific external housing, the descriptions of two of the others they sell say that the maximum drive size is 500gb. My guess is that this is going to turn out to be true for the one I own as well.
So... unless there turns out to be a firmware fix, I can (a) put the 750gb drive into my PC, where it is unlikely to be useful, (b) leave it in the USB casing for loads of backups, or (c) find some other way to make it accessible to multiple PCs - there are gadgets to use a USB drive on a network, for example, but I really don't want to go that route if I can avoid it. There's also an option (d), reduce the partition size below 500gb, but I would prefer not to waste a third of the drive.
What I'd like if I could afford it is a box to take two or more drives and put both the 750gb and 320gb drives into it, which would give me nearly a Terabyte of storage. But my impression is that those things are hideously expensive, and that the affordable ones only take drives to a maximum of 500gb.
So... any suggestions?