This question already has an answer here:
First of all: this is not a duplicate of the all the "GRUB on the wrong hd" questions.
When booting, the system will not boot unless the installer USB flash drive is present. If I boot without the USB drive, the screen goes black AFTER GRUB lists the different configurations available - so GRUB is on the correct disk, its what it does next that seems to be the problem.
With the USB drive inserted, its access light starts flashing while the system boots in a normal manner.
And yes: I have tried booting from the USB drive directly and then I get the "live" version, so I have not installed the OS on the flash drive by mistake :)
I have tried
sudo grub-install /dev/sda and
sudo update-grub just for fun, but nothing changed.
System in short:
Ubuntu 12.04 64-bit. i3 processor. UEFI bios. 128Gb SSD drive. The SSD has a 47 Mb EFI partition and the rest mounted as '/' (XFS).