Error : Please change the battery cover

"The common working solution to fix this dreaded problem, is to reset the mobile phone, this is easy for you to do. Simply power on the phone and enter the reset code which is:*#7370#. Once you have entered the reset code, it will ask you to enter a code which is: 12345. Once entered, the phone will reboot and will be reset to factory settings and should make the hard drive work. If the reset doesn't work, then update or reinstall the firmware on your Nokia n91 that is having hard disk problem, as there could be fault developed in the firmware that can stop the hard drive from working." from here. Hope this helps, good luck.