Comment # 3 on bug 1003616 from Neil Rickert

I'll add my two cents.

My experience seems to be consistent with importing from the most recently used
linux partition.  I had been wondering why the import source changes from time
to time.

And a note to the original bug reporter:  If, by chance, you happen to always
mount that first linux partition, then the mount updates the file system time. 
Assuming "most recently used" is based on file system time, then the root file
system that is most recently mounted is going to be the one picked -- or at
least that's my best guess on how it works.