Key error: foreign ???

[Paolo Brunello]

HI,
I've  repartitioned  the  hard drive using the OS X disk utility and I 
might have done a painful mistake: I made a HFS+ partition for the OS X 
and a UNIX filesystem instead of "unallocated" which I now interprete to 
correspond to the "free space" option.
When I tried to install YDL 3.0, the installer soon hanged up when it 
comes to partitions: neither the automatic partition nor the disk druid 
let me in, both generating a pop-up window saying "exception: this is 
probably a bug" and at the end of a set of strings I read "key error: 
foreign".
I wondered if there is a way to fix this without erasing the OS X 
partition once more.
I booted from the original OS X CD using disk utility again and trying 
to set the unix partition into free space but it warns me that all the 
volumes will be erased.
Is there a way to reformat only the UNIX partition, turnin it to free 
space, maybe using a Linux tool, whatsoever form a CD?

thank YOU,

Paolo