No, they will be of a different density the platters, so there is nothing 'hidden' to find.

Not all platters are the same, even from the same manufacturer. Different products have fundamentally different data densities, so the same size platter fundamentally holds different amounts, and the read/write hardware will be specifically made to match.

It is conceivable a drive could support the data storage equivalent of overclocking. I've seen it done on floppy disks, but reliability goes through the floor, and you might be OK with risking a couple of megs but not several hundred gigs. It's not very likely to be possible on a hard disk though - unlike floppy disks, the controller is built into the unit itself, so not likely to be "hackable".

For certain specific models manufacturers have been known to artificially restrict capacity via use of a host protected area. I had an excelstore 60GB drive which was actually 80GB. I don't think it's a very common practice though, most drives are only capable of holding their rated amount of data.

But these guys are good and have some useful tools so maybe they can help?

http://blog.atola.com/partition-find-and-mount-v22-released/

http://blog.atola.com/restoring-factory-hard-drive-capacity/

Its possible that it was a 160, but the amount of bad sectors made it a 120, so at least they can sell something. I wouldn't try and get access as cost between the 2 is negligible.