To find what is on the drive . I suggest WinDirStat . It will show you everything on the drive . It lists the folders from large to small . You can see all the subfolders and files by expanding each listing . WinDirStat - Windows Directory Statistics
Download and install it . When run click ok in the drive selection box . When done it will show a graphical view of the drive and an expandable list view . You can delete the data you want gone by using the clean up function which is manuel or delete by right clicking on the items in the list .
You can also open the files by double clicking them or right clicking them in the list .
IMHO a realy nice and needed program .
Please be carefull when deleting things .
Oh and you have a 750gb drive . The 3 I have add up to approx 698gb aswell .