Samsung Galaxy S2 stuck on CWM, can't mount sdcard
With the information you've given, it sounds like your SD card might be damaged, or not formatted properly. Make sure the SD Card is formatted in FAT32 with a card reader if you have one available. If there isn't anything you need on the SD Card, try wiping the SD Card through CWM, and reformatting it. If this doesn't work, the card is probably physically damaged.