My iMac is not booting from external SSD after updating the OS

I have an iMac that was running smoothly using an SDD external drive. Today it asked for an update of the operating system, which i did. Now it is not booting anymore, it is struck on this screen: [https://discussions.apple.com/content/attachment/fd5a6793-06e8-49e4-a596-319cd9db8ecf](https://discussions.apple.com/content/attachment/fd5a6793-06e8-49e4-a596-319cd9db8ecf) it is booting in safe mode, but in this case it load the OLD operating system from the hard disk. How can I restore the operating system i had on the external SSD?? the SSD is correctly seen when i boot in safe mode. thanks for any help!