unable to mount hard drives

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http://askubuntu.com – I am currently trying to backup data from my computer's hard drive to an external one while live-booting to Ubuntu from a USB, and I've encountered a bit of a problem. You see, I cannot mount my hard drives. Whenever I try, I always get one of two errors: fuse: failed to access mountpoint [file I'm trying to mount to]: No such file or directory or Error mounting: mount exited with exit code 21: mount: according to mtab, [drive] is already mounted on [file I'm trying to mount to] How can I fix this? (HowTos)