They are.  If the drives are on a backplane and accessible from the front of the machine, they are hot-swappable.  Hot-swap drives are the only option on an R805. Typically, the drive will rebuild automatically, but if not, then you need to assign it as a hot-spare to begin the rebuild.
I have an PowerEdge R805 that is showing a HD fault (E1810). What is the best way to determine if the drives are hot swappable? There are two(2) drives in the server - I assume that they are mirrored and that once replaced the mirror will be rebuilt automatically.
PowerEdge R805 - Hot swappable HD