Thanks for that link, it got me thinking further about these drives. Have been digging around and it would appear that the WD Red series are best designed for use in a NAS. I don't remember them being around when I purchased the NAS and from memory I think I got a deal with a free green drive thrown in as well.
Interestingly enough the Green Caviar variety such as mine are still listed on the Netgear website as being compatible. As mine are still under warranty I am going to attempt to get them replaced with Red drives and offer to pay the difference. Fingers crossed.