Dungeon Siege II, it's quite similar to Dungeon Siege (and they both are good games), but do not consider trying or buying Dungeon Siege III, it doesn't really have much to do with the previous titles.
http://store.steampowered.com/app/39200/
$1.74 right now. Can't really beat that. Oops... accidentally linked DS2, but you can find one from there. 1, 2, 3 and 3 dlc are $4.99 total right now. Great deal.
It's $7 on Steam. Just buy it and save yourself the hassle: http://store.steampowered.com/app/39200/?utm_source=PCGamingWiki&utm_medium=PCGamingWiki&utm_campaign=PCGamingWiki
To answer your question though, not sure about DS2 in particular, but some forms of DRM/copy-protection will check the actual disc for intentional errors. CDs/DVDs have error-checking built in and a bit of redundancy and can recover from minor scratches and laser focusing problems, etc. But smart publishers started deliberately introducing errors onto the physical part of the disc that error-correction normally just absorbs and deals with. So at a bit-by-bit level, the disc looks identical but if you go down to the physical layer (or the error-checking layer), the game can detect those errors and realize they're not the same. https://en.wikipedia.org/wiki/Compact_Disc_and_DVD_copy_protection#Sectors
It's kinda like having the same model/year/color of a car as your friend, but you intentionally etch your name into the tailpipe on yours. Most people wouldn't notice the difference, but if you look carefully you can see your name there... but only if you were looking for it to begin with.