I'm not clear on how WHS writes files to the harddrives. When you copy files to WHS how are the files distributed / what file system does it use? So if one or more drive fails, do you just lose the data on that drive? Is it possible to know what files you have lost?
If not, then you loose all files on that particular drive.
If you have a RAID, it depends on the type of RAID.
RAID0 (stripping) will loose all your data.
RAID1 (mirroring) will not loose any data, as long as you have one drive working.
RAID5 (parity) will not loose any data, as long as you have one drive failed only.
RAID6 (dual parity) will not loose any data, as long as you have up to 2 drives failed only.
AFAIK, WHS is Windows Home Server Operating System.
It's all up to you how you configure your own storage on it. If you have a pre-built system with WHS, then you can check your particular model on how a manufacturer configured it for you (AFAIK, Acer configured theirs as doing a constant file synchronization from one drive to another -- similar to RAID1, but less proof).