Moving /home or /usr (or /var?)
This can be done in multi-user mode. Be VERY careful NOT to use sudo (like I did). After moving /usr to /ousr, you will be doomed (I realized it the instant I pushed Enter). Instead, su. Because of this mistake I had to get off my chair to fix it locally.
mkdir /mnt/nusr mount /dev/sdb1 /mnt/nusr cp -a /usr/* /mnt/nusr umount /mnt/nusr mv /usr /ousr mkdir /usr mount /dev/sdb1 /usr
Edit /etc/fstab, mount -a, reboot to make sure it survives it OK.
Tested remotely from putty.