i use 2 hard drives, a 2 gb for the os and utilities and a 40 for everything else. when i incorrectly tried to install onto the smaller one, i got the disk 5 conundrum as well. no idea what the cause is but you might want to check for disk space and possibly defrag before installing.

wasnt there a problem with the uninstaller not clearing the registry? if you're comfortable with it, run regedit and check your windows registry for any vestigial bg2 files.

I did get it fixed.. I can assure you that between the time it worked and then didn't work there were NO changes made to the settings on my PC, Yes I defraged and yes I had proper disk space, I have BG2 it only comes with 4 disks, No i don't have ToB so it can't be refering to that, No I do not have the collector's edition, no it is not a copy. None of the above solved the issue.

What did solve the issue -

Go to My Computer, Control Panel, System, Device Maganger tab, CDROM, Settings - UNcheck DMA, click OK, restart computer... Then install