I am running Mac OS 9.0 on my iMac, and I just installed a USB drive. Now, my computer stops responding whenever I turn it on.

Possible solution:

You have installed iMac Update 1.0 which is intended for use only on iMac computers running Mac OS 8.1. Running this update on a computer using Mac OS 8.5 or higher will cause the Mac OS ROM file to be downgraded. The Mac OS ROM file included in this update does not work correctly with other versions of system software and will cause the system to stop responding during the startup sequence when improperly applied.

To solve this issue, reinstall Mac OS 9.

If your third-party software installer, such as the one that came with USB drive instructs you to install the iMac 1.0 Update, and you are running Mac OS 8.5 or later, you can safely ignore this part of the software's instructions.

For more information, see the following Knowledge Base articles:

60590: " Mac OS 9: iMac Update 1.0 Is Not Necessary. "
24969: " Mac OS: Computer Stops Responding Because of Wrong ROM File Update "