PowerEdge 6850 Faulty hard drive or PERC controller?

Should read hot "swap".  Yes, technically, hot-plug means hot-add, but that is a specific technology used primarily for add-in cards and memory, and has been traditionally been used interchangeably with "hot-swap" (although I would expect better of Dell documentation).  If the server has a backplane and the drives are accessible from the outside of the machine (even on systems as old as the 2450 (~2000)), then it supports hot-swap.  Also, when talking about SAS and SATA drives, regardless of whether there is a backplane present, the technology supports hot-swap.

Swap it hot ... never power down to replace hot-swap drives (and they ARE hot-swap).  It may turn out alright, but when you power on, the controller has to make a decision which array configuration to use (the config on the new drive or the config on the old drive) ... in most cases, it will choose the right one, but all too often, the incorrect array is chosen (either by the controller automatically or by the user manually).  Shut it down if you want, but just know it is bad form, and you will have earned the big fat "I told you so" you'll get should things go wrong (right before I try and help you fix it) :)