Fixing “E: Can’t mount /sdcard/” Issue
Method 1: Format SD Card to a Different File System
Actually, the “E: Can’t mount /sdcard/” issue is closely related to the file system type of your SD card and the inability of the device recovery system to read a particular file system.
Pull out your phone’s SD card.
Put in a memory card reader and plug it into the computer.
Create a backup of the contents of the memory card on your computer.
Now open My Computer and right-click the drive that shows represents your memory card.
Select Format… option.
Click File system and select NTFS or exFAT or FAT32.
taken from:
How to Fix "E: Can't mount /sdcard/" Issue on Android Devices