Can't boot in OSX (10.0.1) after booting in 9.1


I have got a strange problem. I can't boot back in OSX when I am in OS9.1. I chose the "Starup Disk (9.2.1)" control panel, and check "Mac OSX" volume as startup. I click "Restart"......... and I start in 9.1 again.
The two systems are situated on 2 different partitions of the same volume, which has been formatted with Hard Disk Toolkik 4.5.
I can boot on OSX when I press the option key while booting and I select the "OSX" Icon.
Is it an extension problem (I used to be able to do it before). Or is it linked to 10.0.1 update ?

ThanKs for your help.