Access Is Denied in Drive Wiper

[TheShad]

Running the Pro version on my 64bit laptop. The hard drive has had 2 partitions since new, C and D.

C hold my operating system, D was used for storage.

I'm passing the laptop onto my son and will be installing a fresh version of Windows, but also want to wipe the D drive. However, when I try to run Drive Wiper I get the prompt to insert the word ERASE, which I do, then as soon as I click OK a box pops up with 'Access is Denied'.

I've deleted everything I had on there and CCleaner will happily wipe the free space, but when I try to do a full wipe that box pops up. Any ideas why?