I have an Intel based imac 27" machine which I wanted to upgrade from 10.4.* to Snow Leopard. Using an Apple Snow Leopard full installation disk I installed SL to the computer's hard drive, which seemed to go well but the computer then refused to boot - just grey screen, Apple logo and spinning icon. Tried all the suggested cures but all failed. Safe boot did not work. So Installed the OS onto a USB hard drive. This worked perfectly. Computer will now boot if directed to the USB drive. (pressing "C" at switch on and choosing the usb drive) It will not boot if directed to the internal hard drive. The on board hard drive is accessible and is working fine. So I believe the original Hard drive is working fine. The OS SL seems to have loaded OK but the computer will just not boot from the internal Hard drive. The computer will run using the OS on the USB drive but only the basic start up programs are available. Some programs can be run from the old hard drive but most error (suggesting a reinstall) So basically HELP.