The Nokia n91 is the best mobile phone, when it comes to music, with high end sound quality and has enough storage for tons of high quality encoded music, but it suffers the common hard disk error. If you have a Nokia n91 and you get an error message displayed on the screen that says "Hard drive Unavailable" or "Hard drive not found", then i have the solutions to this problem.
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.