NotReally: Thanks Lucky015.
I'd raised it as an issue shortly after I got VDSL installed back in Feb and that was the answer given by a Vodafone rep. It is a WD Elements Desktop external hard drive with an external power supply. Website says its USB2 compliant so that should mean less than 0.5A? Either way it was crashing the modem but worked flawlessly on a server (and any other PC I plugged it into) so does that mean it is the modem at fault?
Not worried now as I've got a working solution but was interested to see someone else had a similar problem.
Cheers.
If it has the WD tools and backup stuff that runs on the drive when you plug it in remove those and you should be good.