[ubuntu] help with navigation

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http://ubuntuforums.org – I have a brand new fresh installation of Ubuntu 12.10-amd64, which is acting suspiciously flakey. I would like to read the md5sum of my .iso file & check that against my source file. Yet my source file takes up 8 GB on my USB DISK, and was never in the Downloads directory. I downloaded the .iso file twice and - under Windows - ran fc (Similar to diff) on both images. They were identical, and most likely can only have failed in the disk burning opertation, which was done on my Ubuntu hardware. Thus, as near as I can tell, I must find these files within the 12.10 filesystem and run md5s (Hardware)