HDD recordings freezing OS Mega


Hi all. I've got an Edision OS Mega running Willo's build with a 500gb internal HDD.
First noticed this problem Wednesday night, every programme I record to the HDD I have problems with freezing/blocky pictures, I lose about 8 seconds then whatever's recorded continues until the same thing happens again. Got so bad tonight I gave up after 15 minutes and just deleted.
HDD was new shop install i.e. not some old 2nd hand rubbish I had lying around :)
Also should say signal quality isn't in doubt, there are no problems watching live.

Any suggestions as to what may be the issue, before I go blundering in, will be gratefully received, thanks.