I had an XP System and I installed Windows 7 on another Hard Disk so now I have a dual boot. However, when I boot windows 7, the hard disk where XP is installed does not always show up.
This started after few weeks where everything worked fine.
So the XP disk doesn't show in explorer on Windows 7, and in Disk Management Utility it is marked as Unallocated. When I reorder HDD boot order in the BIOS, it seems to fix it from time to time, for a few boots, but it gets back to that bad state after some short time.
I have a Gigabyte PS3 motherboard and I updated the IDE/Stata controllers for Windows 7. HDDs are Western Digital and Seagate. I checked the SATA cables; they are correctly plugged. The power supply is 700 Watts up and working.
Edit : Similar Issue here