Mounting on top of existing files

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http://www.linuxquestions.org – I've got a multiboot. They each mount each other's partitions. By accident I had existing files on one of the mount points that another partion mounts as home. When it's used as home on that one install you see its proper home directory data and you can't see the files that were originally there. When I boot back to the other install, you see the original files but not the home directory files from the other install. Nothing gets lost. Where is the data from one install when the other install is mounting its partition? (HowTos)