The 20 missing gigabytes on your iPod is not all firmware. Most of it is a well-known marketing ploy. The GB they quote on the box assumes that it is 1E9 (1,000,000,000) bytes. Computers define bytes in binary, so it's not a nice number like that; an actual, computer defined gigabyte is 1,073,741,824 bytes. Your iPod's 160 GB ends up being formatted at about 149 GB. The rest is taken by firmware and other low-level data. 120 GB for the Zune ends up being about 112 GB after formatting.
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