Replacing of hard drive
Location, replacement, and largest size that can be on tablet on the Asus Transformer T100
Released September 2013, identified with model number T100T. 2-in-1 portable with Quad core Intel® Atom™ processor and 2GB of RAM.
You cannot replace the "hard drive" because it doesn't have one. It has an integrated chip that's solid state. What you can do though, is use a micro sd, and get a OTG (on the go) connector for your micro usb, and connect tons of usb thumbdrive sticks to it, or an external HDD.
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.