building summary file in thunderbird

I am using thunderbird 1.0.6 under two OS at the same time: Win32 and 
Linux. I have set the Mail & News folders on a shared disk partition so 
I can see & write into whatever the OS am running. The problem is that 
when I restart from one OS to another and launch Thunderbird, a message 
in the status bar tells me "building summary file for INBOX toto@...." 
and takes sometimes up to 10 minutes (large number of mails)
WHAT's THIS ? and how to get rid of?