Windows is going to assign drive letters in essentially alphabetical order. The OP appears to want to assign the external drive to W:, which is a good idea as that drive letter will not get assigned. Therefore, a USB connected raid box can reliably maintain that drive identifier.
Some may argue that if you connect enough drives, than even W: could get auto assigned? However, in WIN 7 Pro x64 in enterprise environments, we've seen drive combo limits where WIN will no longer recognize additional connected drives. Therefore, W: is a safe assignment to choose.
BTW, if you never dealt with external raid boxes, many now are pretty much brainless to setup. The firmware takes care of everything. Can be as simple as plugging in and formatting the drives, and your ready to go. Depending on the manufacture, you don't even need to use any software.
