E:unable to mount/usb storage is a standard error in TWRP, it always shows as it's not supported.
As for your zip not flashing and failing, try a different one, or re-download it.
The people of XDA, including the Devs. Do not have to help you. We all choose to help. Everyone also starts somewhere.
City I live in makes and invented the Kitkat... I run Kitkat on my device. Aww how cute.
CM11 N9005(Note 3) - Nexus 5(D821)