I'll take a stab at it.
Do you have different filesystem types on the nas and the usb drive? That's the most likely cause as you can have different files take up different amounts of space (even on the same filesystem type actually.) This is caused by the way the files are physically stored on the drives, basically if a file or part of a file is smaller than the block size of the device it will take up the entire block. Ex a zero file (touch ~/test) will still take up one block on the storage unit (also a bit of space for an index but that's a little to complex for me to be able to explain it easily or quickly.) So that would explain why you'd have two different sizes reported on the drives themselves. I wouldn't worry about it to much but if you want to verify that the filesystems contain the same files it wouldn't be to hard to write a shell script do a directory tree of each drive and a quick diff.
I'm not so sure about the nas box as I haven't used that model, but my first guess would be that whatever operating system is running on it is stored on the hard drive and not a flash chip internal to the device.
Edit: After looking at your post again you seem to have more data on the device than a small os would use. Are you sure no other shares are hanging around on it? I don't know why it would have so much more space used really.
Last edited by travisdh1; 08-17-2011 at 11:47 AM.