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.
// Nexus 5 (D821) // Liquid // UBER //
Note 3 (SM-N9005) // AOSP ROM of variety // Lean or temasek's Kernel // - Dumped. No more Samsung