I was transferring some recordings from my EX-77's hard drive to DVD-RAM last night when suddenly it decided to give me a U88, warning that the last recording may not have worked because of dust etc.

Since that it won't recognise or play DVD-RAMs or commercial DVDs. In fact, it doesn't seem to know whether there is a disk inserted or not - telling me the disc it's not recognised either way.

I also get a lot of clanging noises from the drive as it tries to read it.

The hard disc is fine.

The machine is five years old now and I was wondering if, aside from a repair, an option would be to buy a second-hand E77 with a working DVD drive and put the hard disc from my machine into that?

You will need to clean the spindle seat that the disc sits on, take the top off of the machine and expose the spindle, there is a rubber washer at the base of the spindle, clean it off with some white spirit soaked onto a cotton bud, keep stroking until you get a clean bud. Allow to dry and rebuild. That should sort out your problem.
Dave