[Q] E: Unable to mount '/sdcard' ?

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http://forum.xda-developers.com – I got the latest TWRP Recovery (v2.6.3.0). But it says "No OS Installed" what does it mean by that? How can I fix it? Actually when I try to do anything like wipe something I keep getting these errors: Code: E: Unable to mount '/sdcard' E: Unable to mount storage. It seems that it can't locate my sd card. Although when I press mount I can see all the files on my computer but I can't see it on the TWRP's File Manager or when I try to install a .zip (HowTos)