How to multiboot ubuntu 11.10 with another linux? [migrated]

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http://serverfault.com – I have installed centos on my /dev/sda9. There is windows 7 on my first partition. Now I try to install ubuntu on /dev/sda11 While installing,it shows the following error. migration-assistant needs to mount a partition,but cannot do so because the following mount point could not be mounted: /dev/sda9 please close any application using this mount points. would you like migration-assistant to try to unmount these partition again? How do I address this error? (HowTos)