We format with a low number of inodes (1 per 128kB), so these overheads aren't too large. The main culprit is that the drives are 20GB (is 20,000,000,000 bytes), which works out as 19.073 "real" GB (ie, 2^20).

A 40GB unit is then around 38,146MB without the space being taken for operating software (80MB per drive) and database/inode space on the music partition.

Hugo