Multi os boot system, one disk won't boot (well, in a way!)

Yes well I could but wondered if there was corrupt boot info on the 320gb that I could possibly fix since thats the disk I boot from most of the time now.

Thanks space1 too. You have actually explained why in an easy to understand way! So the master boot record (MBR) can only be present on one disk in a system right? I first installed win 7 on the 300gb hdd a few months ago after I had upgraded an 8 year old cpu, motherboard, ram etc to i7 2700k but I didn't buy a new pc case but plan on doing soon. Basically the 300gb hdd (Maxtor Sata I 150mb's) is also 8 years old and as my system doesn't have enough room for all the hdds I've acquired with proper ventilation, the temp. of it goes over 45*c while the rest of my hdds stick around the 30*c and are much newer (and also quieter and faster both sata II and III's) but as my case can't hold more than 3 hdds I have to put my 300gb disk into a spare 5" drive bay loosely. I have also had two 1tb hdds go raw (not at the same time) though still useble (after a format) and data is recoverable and so as I've only had 3 sata data and power cables I wanted to try and boot into W7 on the 320gb disk, disconnect the 300gb and plug in both terabyte drives so I can backup the salvaged data then reformat the corrupted drive and start to use again. I don't like using the 300gb drive while the system is booted from it as it makes a noise when accessing data, starting up etc and also there is much more apps installed on there so I just use it just incase I can't get into the other win 7 install. That way I can guarantee that I can get into my pc either way if needed. Before I upgraded my system last year I had a nice 3 year old WD 500gb drive that had xp on it until Acronis Disk Director messed it up and all lost files (though root folders still appear just no contents in them) when I wanted to shrink the partition. When I get to finally savage data from that I will put a fresh win 7 on it and do away with the 300gb disk altogether and use it just for extra storage. Sorry for the long post but you asked and I felt I had to explain the full situation! Actually thats not the whole story, one of my 1tb disks (2 year old very reliable Samsung HD103SJ) went raw at the same time I was trying to fix my monitors DVI-input EDID corruption recently. Still not fixed that so vga will have to do for now. The end!