Ok it looks like no one can help - the only additional piece of info I have is that I changed the setting so that it displays all external divices on my desktop - but this seems to have made no difference.
I know there are issues with reading an FAT/NTFS format - but I would expect the mac to at least find the hard drive
Err... are you sure about that? For a drive that's plugged into a router via USB it seems like it would matter. I know true NASs are format-independent, though.
It would make sense that you'd need some software on your computer to manage access to the disk, otherwise two computers could access it at the same time and make a mess of things. There needs to be some kind of synchronization.