can't mount sd card, can't mount back up on canvas a300

Hi guys!! I have rooted my phone ( canvas a300) through King Root. I flashed CWM recovery with the help of Mobile Uncle Tools Mtk. I tried to install custom rom lollipop through rom manager but it prompts " can't mount sd card". I tried to do a nandroid backup but it prompts " can't mount backup path". Basically i am not an expert in such cases. Can u guys help me out? Want to install custom roms for better performance.