Disk Management

Well, I don't understand why that display is showing the extra "unallocated" portion, except that maybe the driver that supports the external disk is capable of providing additional space in some other disk format.

In my experience, all disk drives today are sold with a bit of misleading advertising. When they say a disk is 40GB they almost always mean that the usable space will be approximately 40,000,000,000 bytes. (Same with 20GB, 100GB drives etc.) Now the terms 1KB; 1MB; and 1GB technically translate to: 1024 bytes; 1,048,576 bytes and 1,073,741,824 bytes, but usable space in disk drive sales never translate to this much space. It's just how they are marketed. If you truly had usable space of 40GB that would be 42,949,672,960 bytes.

The image I attached shows my embedded 40GB disk (all allocated to a single partition). The sizes you see there are very close to the usable space you are getting with your external drive. (It is a little disappointing when you buy your first large drive and the usable space is less than you thought - until you find out that this is the way all these drives are sold.)

I can't imagine that you'd ever be able to use more than 37.2GB from a "40GB" drive.