If you are positive that there is a valid root file system on /dev/hdb2, then there is a way to bypass gparted's check. Before you run the installer type the following from a terminal:
sudo vi /usr/lib/ubiquity/ubiquity/validation.py
Find the lines that say:
if not root: result.add(MOUNTPOINT_NOROOT)
And change them to say:
if not root: pass
Save and exit the file and run the installer. Please thank the good Ubuntu users of this thread. They are the ones who discovered the solution.
This was first published in May 2007