Let's see if I can get rid of some of the nonsense, conjecture, and random statements about this from people that don't know:
The problem is drive manufacturers use decimal instead of binary.
(80 * (1 000^3)) / (1 024^3) = 74.505806
An 80GB drive calculated in this way should be slightly less than 74.5GB after format.
Now, as for why your drive is 76.5 and not 74.5GB, as I've indicated, there is a very simple reason.
Your USB drive is *waves magic wand* a Retail Maxtor hard drive (if you were to crack open your external drive's case, this is what you'd discover). Maxtor has made many 82GB drives (retail) marketed at 80GB, while their OEM drives are actually 80GB. This accounts for the discrepancy in the math. I suspect we would also find that your size is closer to 76.3, not 76.5, and that you rounded or mistyped?
(82 * (1 000^3)) / (1 024^3) = 76.3684511
Here are a few more common reductions:
(74 * (1 000^3)) / (1 024^3) = 68.9178705 (74GB = 68GB)
(160 * (1 000^3)) / (1 024^3) = 149.011612 (160GB = 149GB)
(250 * (1 000^3)) / (1 024^3) = 232.830644 (250GB = 232GB)
(320 * (1 000^3)) / (1 024^3) = 298.023224 (320GB = 298GB)
A quick analysis will show that current drive sizes are actually reported about 93% of the size that they are marketed. In other words, you can make a really rough estimate by reducing the size of the drive 7GB for every 100GB. i.e. a 500GB drive would be a rough estimate of 465GB.
(500 * (1 000^3)) / (1 024^3) = 465.661287