I am trying to upgrade from Tiger to Snow Leopard. I'm using a .cdr file on a USB drive to do the install
I'm prompted to restart the machine to continue the install, but nothing happens after the restart.
Burn the contents of the .cdr image back to a DVD. To do this, insert blank DVD-R or DVD-RW, launch Disk Utility (located in the folder Applications > Utilities) and press Cmd-B. Then select the .cdr file in the file selection dialog … That's the easiest way.
But if you have no Mac with DVD burner at hand, you will need to create a bootable USB flash drive (at least 8 GB capacity). You will have to use Disk Utility to first re-partition and format the flash drive with the 'GUID partition scheme' and the 'Mac OS X Extended (journaled)' format (HFS+, journaled). This makes the flash drive a bootable medium. Then use the Restore feature of Disk Utility to copy the contents of the .cdr image to the drive.
Alternatively, you can also buy the Snow Leopard DVD which Apple still sells for $20.