/dev/sda is in use -- from liveCD

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http://askubuntu.com – I'm confused-- I booted from a ubuntu cd and selected the "try ubuntu" option. As I understand it, this is the new "live cd" style. I then try to check my HDD using fsck and it says that /dev/sda is in use. I think, ok, well let's unmount it and issue umount /dev/sda/ -- and it says /dev/sda is not mounted. What's up with that? I thought that if the disk was unmounted then how could it be in use? Why can't I check it? Thanks (HowTos)