The internal storage is divided up in two partitions, a few GB for the system and the rest for storage. So when you connect your phone as a disk drive, it will appear just like any phone with an SD card would.
The LTE model only comes in a 16GB capacity.
My One X has 2.21 GB for the system and 9.93 GB for storage (total of 12.14 GB).
HTC One [UL / Stock JB 4.1.2]
HTC One X [LTE X325a / Retired]
Asus Transformer [TF101-B1-B60 / Retired]
HTC Desire [GSM A8182 / Retired]
HTC Hero [GSM Black / Retired]