Strange Problem-USB Now Primary Disk

[Warpig]

I built my pc and had a dual boot going on with XP and Vista. I took my pc apart this evening to fix a fan. Plugged everything back in, "boot disk failure insert system disk and press enter" popped up. My HDD usually plays around with me a little bit so I wasn't too worried. Checked the connections, and all the basic, still the same problem. So I ended up popping in the Vista disk and deleted that partition. Same problem after that. So I put the XP disk in, and fixmbr and fixboot... same problem. I decided to just unplug my usb 500 gb WD HD, I forgot why, but I did. As soon as I did that my PC booted up in XP no problems what so ever. (wasn't expecting it after I just deleted Vista since it has the bootloader) So I plugged my USB drive in again, popped up all normal. Restarted it just to see if that was really the problem. It was, same "boot disk failure insert system disk and press enter" error. Unplugged, booted up again in xp. I looked on Partition magic and on Disk management, both say that I have two primary disks, my 320 internal and my 500 external. So now when I boot up with the external plugged in, it tries to start windows with that one, even though it doesn't have the OS on it. Any reasons why this happened all of a sudden? Any ways to make my external logical again without reformatting? Thank you in advance!

[Warpig]

I just entered bios and had HardDisk as first boot and then CDROM and I disabled other boot devices, and this seemed to work. Just odd it happened all of the sudden.

[mjdodd]

I think maybe youve solved it, but I had asimiliar problem when I took apart a machine recently to find that he hard disk cables had been connected wrongly o the motherboard... ie the first hard drive slot had the cd & DVd and the hard drives were on the cd slot... I suspect it may depend inmy case on the motherboard.
bOL
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[Thinka]

I just entered bios and had HardDisk as first boot and then CDROM and I disabled other boot devices, and this seemed to work. Just odd it happened all of the sudden.

That's what I thought would be the problem. And as an aside, you might want to have your CD Drive as your first boot device - for that day that you want to do a recovery/re-install u know?