I re installed Ubuntu 12.04 from a flash drive, it doesn't boot without the flash drive.
I tried to install the grub in using the command sudo grub-install /dev/sda but it says it cannot find the device for /boot/grub and asks if the device is mounted
I checked with the disk utility program, the disk is there, its name is sda1 and it is mounted.
What could be my next move?
It sounds to me like you probably mounted the boot point for Ubuntu on the Flash Drive rather than your hard disk or the file system. If you run the setup again, from your flash drive, and repair the current installation. It should cure your problems, although, you may need to reinstall if a repair doesn't fix it.
I could be talking a load of nonsense as I have not had any problems with booting, so check first that there are not any better looking at posts like Can't boot without Flash Drive plugged in
