I don't use Clockwork any more, but i did for almost a year. Here's my notes.
Clockwork Notes:
The first thing about Clockwork is trying to get Rom Manager to flash a good copy onto your phone. Here is a fool proof method to determine whether you have a good copy before trying to reboot into it. Begin by scrolling down and flashing the alternative recovery – SpRecovery. After it successfully loads, scroll up and flash Clockwork. If it takes longer than 3-5 seconds to get the popup telling you the install was successful, reflash it over and over until you do get that popup in 3-5 seconds. Once that happens, you have a good install.
Once you have rebooted into Clockwork and are navigating through its menu, if at any time it hesitates that means it is about to reboot automatically. When Clockwork does reboot and the screen goes dark, immediately press and hold the X key until the phone reboots back into Clockwork. Clockwork will now be totally stable until you finish.
If the first thing you do with Clockwork, during any particular session, is to try and make a Nandroid backup and you get an error message (without regard to what it says), you should just wait a few seconds and not do anything. That is because Clockwork is about to reboot the phone. As stated above, as soon as the screen goes dark, press and hold the X key until the phone reboots back into Clockwork. Then start over and Clockwork will work as it was intended for the rest of the session.
