in the last part of chapter 10, in 10.c you have the commands # exit # cd # umount...... the cd needs to be first, otherwise you get an error if your in one of the directories your trying to umount, I'm new to the gentoo install and it took me 3 tries to figure out why it kept saying /mnt/gentoo was busy. putting the cd first brings to back to the root where when you hit exit it actually logs you out and puts you back in the install cd env where your free to umount everything.
i think you ran some commands in the wrong order the "exit" is to get out of the chroot (typically /mnt/gentoo) the "cd" is to make sure you arent in any of the /mnt dirs by moving you back to either / or /root (i forget which on the livecd) since your $PWD is now / or /root, doing umount on /mnt/gentoo points should not be affected by the active shell
Comment 1 is correct, you have probably left some other shell in another console.
