Bad HD required to use good HD

Ok, I consider myself fairly good with computers but this problem just baffles me.

My setup: I have 2 Hard Drives, both Western Digital. The old HD (about 8 years 320gig) has 2 partitions (C: contains only program files and Windows XP, D: contains media, documents, and games. The new HD (about 1.5 years, 750gig) contains Windows 7, media, and games. I only keep XP as a backup in case of weird problems with 7 and I haven't really used XP in over a year. Theres no coupler on either of the drives, they simply share a power cord and use different SATA cables.

I noticed my computer running slow and videos running choppy (They would stop altogether, then start again.) So naturally I think some background program. After all the programs were shut down and processes that weren't essential closed I was still having this problem. So I did the natural thing to run a virus scan, then a malware scan, then a HiJack This log. All came back negative for malware... etc. I realized movies and music worked fine on my newer drive so I figured the 320 was just crapping out. So I think "No biggie, I'll be fine considering all my essential files for running windows 7 are on the new drive."

After a windows update I was forced to restart and then I was greeted by a "Disk boot failure - Insert system disk and press Enter" message and the "Click of Death". After fiddling around with BIOS settings I found that the old HD had been selected (somehow) as the HD to go to in the boot order for HDs. So I selected the newer one to boot from by hitting F8 at the BIOS screen and selecting the 750gig... now here's where it gets strange...

Starting from the 750gig drive appears to give me the same disk boot failure message (even though I'm certain that drive is physically fine.) If I select the 320gig drive it will load a menu asking me which OS I want to start from (Win7 or Earlier version) Selecting 7 will simply give me the error again and the earlier version will just spew out error messages and crash. If you run the "last known good configuration" option it works fine. The 320 is viewable but laggy, sometimes programs crash when they read files on that drive.

I unplugged the old HD because I had made backup copies of files to the new HD. (All media and documents, no programs) But I still recieved the Disk Boot Failure message. The only way I've been able to access windows 7 is by plugging the BAD HD back in! Even though it shouldn't affect the Win7 OS because its on the other drive! (ARGH!)

I plugged in the HDs in other SATA ports and even used some spare SATA chords I had, and nothing seemed to remedy the problem. Sometimes I wouldn't even be able to ask the BIOS to load the last known good config for win7... it just keeps popping up with that message.

It's like a nintendo cartridge, eventually I can get on and everything within the newer drive works fine.

So, after all that, my question is WTH?! I could understand HD failure, I could understand MOBO failure (mobo is only about 3 years old) But both at the same time?! I don't have any of my windows backups (and I'm currently looking for a bootdisk for either my cd\dvd player or my thumb drive. What do I do now? Why is the new HDD acting fine when I DO finally get into windows but gives me all sorts of grief at the bios level?

I have an ASUS PDN-5 if that helps. I don't know if it should be in this forum or in a Mobo forum considering I really don't know what's causing it.

So basically: Why can't I use my good HDD without plugging in and attempting to boot from the bad one to get a menu that will let me load a configuration so I can use windows without any problems?

*pant pant*