unable to mount storage while installing new ROM - Nexus 5 i have nexus 5 rooted, its version was 4.4 i tried to upgrade it to 6.0 so i download the ROM (image-hammerhead-mob30y) from developers.google.com and i copy the image to the phone by adb push command and when i try to install the image i get the error unable to mount storage. however i cant wipe internal storage or dalvik cache from the wipe section in TWRP. also every-time i try to get root access to the shell using 'su' command i get the error /sbin/sh: su: not found

so what can i do to install the image? if the issue with the image what did you recommenced?

thanks
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.