Hey Guys, I've been searching for days on how to fix this problem, so I'll be very clear with what I have already tried. But first let me describe to you my situation: Mac Pro 2006 (Intel Based) Used separate hard drive (internally) to install Windows from an OEM CD with Service Pack 3. Install went fine, everything worked as usual. I could hold option on startup and choose my OS. Few days later, I moved my Mac Pro from my home to my work, and when I started my Mac Pro, holding the option key of course, it went straight to Windows XP. I thought that was odd, so I tried a restart, and again same thing. I took out the hard drive with Win XP on it, and restarted computer holding option - this time went to a black screen that said "No Bootable Device". Then I tried holding CMD + OPTION + R + P to reset my PRAM (on a USB keyboard to assure connectivity). No luck. I fiddled with my drives, took them out, put them back in, and randomly it worked! I held alt and chose my OS. Snow Leopard started up as if nothing ever went wrong. I then re-started and booted into XP totally fine. Went back to OSX and everything was 100% good. A week or so later, after unplugging my Mac Pro, and re-plugging it back in - same problem. Option key won't let me choose OS, and when I take out Win XP hard drive, I get the "No Bootable Device". My guess is that it thinks it has to start into Windows XP when the drive isn't there, but I don't get why it won't work when I simply hold the alt key. Unfortunately, since I installed straight to SP3 - I can't install Bootcamp on windows side to tell it to boot to OSX. I also believe I can not downgrade. Anyone?