Some addtional info. Just so this is posted as also, but I had more permissions on this site I would start a thread about it. I discovered today that on my crappy 4 year old Dell Laptop (Win 7) Arma tools will NOT mount a P drive if I'm running Steam in administrative mode. It's kind of weird, but could be a dell thing, but i've noticed some people having trouble mounting their P drive, see here:
http://forums.bistudio.com/showthread.php?178847-DevP-not-Working-%28Windows-8-1%29&p=2711969#post2711969