unable to mount storage while installing new ROM - Nexus 5
You don't install factory images in custom recoveries, let alone &quot;flashing&quot; them like regular custom ROMs.
XDA has you covered for simple stuff like this. Excerpt taken and simplified below:

Install adb and fastboot drivers.
Download the latest factory images
Extract the archive you downloaded.
Unlock your bootloader (In case you haven't already)
Power off your phone.
Boot it in fastboot by pressing and holding Power Button + Volume Down at the same time.
Connect your phone to your PC through your USB cable.
Execute the flash-all.bat script (for Windows) or flash-all.sh(for MAC and LINUX) in the folder where you extracted the factory images.
Wait for everything to flash.
After everything finished, select &quot;Recovery&quot; using the volume buttons.
When a small dead Android appears, press Power Button + Volume UP.
Select &quot;Wipe data/factory data reset&quot;
Reboot.