Am I right that you have the 2 on-board IDE controllers (primary and secondary) supporting 4 IDE devices, plus an additional controller card to connect the remaining 2?
And further, the "missing" drive is connected to the additional controller card?
If so, your suggestion to connect the "missing" drive to one of the on-board controllers is good - you can't do any harm, and you may well get the data off.
Are the drives formatted with FAT32 or NTFS? Shouldn't be an issue for this, but if you need to put the drive in another system it may make a difference, due to file security.
I'm a bit puzzled as to the "upgrade" from XP pro to XP pro with SP2 - was this a re-install or just installing SP2?
SD