They asked for a copy of the data on an otherwise clean hard drive, likely because they want to assess what he copied. From the letter, it's clear that they do

They asked for a copy of the data on an otherwise clean hard drive, likely because they want to assess what he copied. From the letter, it's clear that they don't expect anything else to be there, since they comment that they're going to wipe the drive when they're done looking at it, before returning the drive. The drive is just meant as a convenient way to transfer a large amount of data.