[ubuntu] Can't switch back to Mac OS X

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http://ubuntuforums.org – Hello I'm an intermediate level user and may be the cause of my own problems. I'm using a Mac Pro 1,1. rEFIt installed but buggy. When I could not get rEFIt menu at boot I opened Mac System Preferences and in Startup Disk found the Ubuntu disk. I selected this as my startup disk and since then have had several hit and miss sessions with Ubuntu in tryout mode (quite buggy but most things can be made to work). When I shutdown and reboot the Mac always starts in Linux mode. There is no eRFIt window and if the Ubuntu install disk is not installed an instruction to insert disk appears. I have trie (Hardware)