in the setup program , at thepoint after fdisking , when you are choosing the mount points for your linux system , i found i was unable to choose some names simply because they started with a recognised system dir name...ie /binaries /devices /development /binwibblewibble etc. Currently don't have time to look at it myself to suggest a fix. Sorry!
Yes, you can't make a /binwibblewibble mount point :-) Any mount point that begins with /bin /dev /usr /etc etc. will not work. This procludes you from making /development /etcetera /usrfoo etc. Mount points that begin with /boot, ie /bootfoo, work.
This issue is resolved in the 6.1 installer.