I just tried to update my Z1 with the CNDV-900HD disc (2009 maps) (previously, the 2008 maps were successfully installed) and the system only gets to 26% and then says "An error occurred and the update failed. Update is restarted by pressing "Retry"".

I have tried "Retry" dozens of times. No luck and the system is completely inoperable becasue it wants to complete the update. Any way to reset the system while I figure this out?

You have to keep hitting retry it will eventually complete the install. Most of us on here that have done the update had the same problem. Keep hitting retry until it says complete.

This is a known problem when upgrading from the original Z1 software. Like mdmill1 stated, just keep hitting retry.