Thanks AF. There's a chance it was caused by my RPi, I'll keep an eye out and see if it happens again. I've just swapped the Disk out today for a 1Tb (from a Virgin Tivo off fleabay, which in my case turned out to be a Seagate Pipeline btw) so if it was a disk error that will be gone now. However I did run some tests on the old disk with no error. Like you say, maybe just one of thise things.