I decided to obtain a serial lead to see what was happening during the boot process and bingo the unit booted at the 1st attempt from an usb image. At present I have manually copied the live usb image onto a hard disc as the script failed, changed the fstab file and rebooted the machine. The following forum item appears to be the same problem “can’t boot when serial cable is to powered off pc ". If the update is successful I intend to solder a pullup resistor to the serial Rx lead to prevent it floating.

Andy