How to access phone internal storage during flashing?
From the steps above, I think you already flash a clockworkmod recovery (boot.img)


HOLD volume down until the recovery shows up
Use the volume keys to select Mount
Select mount USB storage
Connect your phone to your PC
Copy the CM10 ZIP file to your phone’s internal SD card
Eject phone from PC and unmount USB storage
Select Wipe data/factory reset > Yes.
Select Install ZIP from SD card > Choose ZIP from SD card > select the cm10_endeavoru-ota-eng.noeri-028.zip file > Yes.
Select Go back > Reboot system now.