Hi there macca45,
It would be nice if you provide some more information.
How many physical drives do you have?(you can check that out under Disk Management) In Disk Management, you can check out the free storage space.
What are your system's specs?
Apart from that, I believe that it may be a good idea to test the drive with some of these:
http://www.tomshardware.co.uk/forum/282651-32-best-diagnostic-testing-utility
Also, you can open Resource Monitor, right click on any of the drives and see if you will get high usage of something, while the screen blanks.
Some screenshots may be a of help as well.
Cheers,
D_Know_WD