Iomega external drive won't show all files!

Hi, I hope someone can help me here. I've had an Iomega external drive (150G) since 2007 and have used it without any issues until recently. When I insert the drive into my new laptop, presario cq62 (windows 7), it shows me about 72 items but not the rest. It opens on some computers but not others, including mine. I have a lot of folders, each with contents...neither the folders nor contents are showing. However, when I check the size of the drive, it tells me 75G of space is free, which leaves me to believe the folders and programs I can't access are stilll on the drive. I have cleaned the drive with Norton anti-virus software which has removed any infections but I still can't access the remainder of my files. At first I thought it was some virus but if that were the case, why are some files (72) showing and others missing? And if my data has been lost, why does the drive report that there's half as much space available? The 72 files showing aren't heavy files at all. Some are word documents and others are folders with light content. I've checked disk management and the drive is reported to be active, healthy and working properly. I would appreciate some helpful feedback from you all. Thank you very much!

Are you certain the harddrive hasn't been split into 2 seperate partitions?
If you right-click on the "my computer" icon on your desktop, or in your start menu, and choose "manage", a screen called "computer management" should show up. From here, choose the option "disk management". (See attached screenshot)
In here, please expand the lower-central part, so that there's no vertical scrollbar, create a screenshot of this screen, and attach it to your reply.

If you are certain that no seperate partitions exist, you could try to use WinDirStat to show you exactly what amount of diskspace is being used by which files.