Dustin is correct for most Asus T100. But Asus does produce a version with a 500GB hard drive installed in the keyboard unit. The standard unit, without the hard drive, is likely a T100TAM-C1. The unit with an installed 500GB drive is a T100TAM-H2.
I am not 100% sure, but I believe all keyboards are wired for the hard drive, regardless if one is installed, or not. So a hard drive can be added to a unit after purchase.