i'm having problems with my computer so i'm going to send the HDs to WD for repairs. if i use the advance replacement (http://websupport.wdc.com/warranty/rmainfo.asp?custtype=end&lang=en) where they send me a replacement drive first before i have to send them the bad drives, what if their tests find that HDs rn't the problem? will they charge me right away or what? first time having to deal with hardware problem with WD products, an any helpful advise would be greatly appreciated on how i can have a smooth replacement.

Not sure about WD, but most companies will usually charge your credit card for the replacement hardware, until they receive and test your defective hardware then they give you the money back

So just make sure your drives are really dead, test (if possible) with different boards, PSU and cables.

I always used the advanced RMA and never got charged, but I also haven't had to RMA in a few years, knock on wood, they will however charge you if they don't get the old drives within like 30 days or something like that. Don't know about if the drives aren't actually damaged though. Just run the WD tools on them. You can get them here