The hard drive is not reporting the correct size
Hard drives use a table to map out the hard drive. This table is called File Allocation Table (FAT).
When you look at the size of the hard drive, this portion is not reported. Because of this, you may think your hard drive is failing or you didn't get the right size hard drive. For example, if you have a 40 gig hard drive, Windows may report the size of hard drive anywhere from 39 gigs to 37 gigs. As mentioned before that is because the FAT table is not reporting the amount of space it uses.
The amount of space for the FAT table is not a set amount. The bigger the hard drive, the bigger the FAT table will be.
If you feel your hard drive is not functioning, download hard drive diagnostics from the manufacturer and run it.