BleepingComputer.com: Hard Drive Storage Space

Why is it when you buy a new HD, say 100GB, you can only actually use 90 something of it? Why is that.

1. There is a difference in accounting for bytes between how the OS counts them and how the vendor counts them. The former counts 1 MB as 1,045,576 bytes, the latter as 1,000,000, so you "loose" the difference with each MB of memory.
2. There is an overhead that the computer uses to manage the disk which is subtracted from the original total as well.
Regards,
John