Win 7 64 partition lost

My system is an ASUS P5B Deluxe MB with IDE and SATA controllers. I installed win xp x64 on the IDE boot drive in 2006. This drive was connected as the primary drive on the IDE controller along with a CD drive. I subsequently partitioned the drive to make room for win7 64 and ran it as a dual boot system. Win 7 quickly became my OS of choice and I disabled the dual boot. On Friday I disconnected the boot drive to test a few old IDE drives I have in storage. When I was done I reconnected the boot drive and it booted in XP x64 and the win7 partition is not available. Disk Manager shows only a single 250 GB partition.

ASUS support suggested to clear the CMOS and install the default BIOS setup. That did not work. Will EBCD help me in this 64 bit environment? Will I damage my chances of recovering the partition if I run ECBD on the XP OS?

(I have a complete win 7 backup history on a remote NAS and all my data is on a RAID 5 that is connected to the SATA controller and also backed up on the NAS.)

Thanks for your help.

How big is this drive ?
Are you saying that a W7 partition on the end has become invisible, or that the XP partition mysteriously expanded to obliterate another OS while it was sitting on a shelf ?
Are you 100% positive that you're not accidentally working with a different HDD, one on which you had a previous iteration/clone of your original XP ?
I can't imagine any way in which W7 can have disappeared, and whilst installing EasyBCD on your drive won't do it any damage (it's just a passive app (like regedit) which does nothing unless you use it to make some changes to the BCD), it's not going to help at all either if there's no sign of W7.