from Jan Axelson's USB Mass Storage book:
"In the data-transport phase, the device returns a structure
that contains the LBA of the last block in the media and the number of
bytes per block. Note that the command requests the LBA of the last block,
not the number of blocks in the media. The first LBA is zero, so the LBA of
the last block equals the number of blocks - 1." 

I may be wrong - I haven't yet got this library up and running.