Replacing WD20EARS in RAID configuration

Hi - I have two network file servers set up using Synology DS410J, each filled with four WD20EARS hard disks.  I need to buy more replacement disks now, but I see that apparently the WD20EARS models are no longer set up for RAID.  What models would be recommended for replacement disks now?  If one of the existing drives fails, can I just replace it with the new model?

Tim Howells

what raid environment are you using? raid 0, 1, 5, 10?

Sorry - RAID 5.