Off topic , however should be pointed out , if you move the physical paths of the users home folders , any users scripts that have hard coded paths in will most likley start to complain about things not being found ..
My preference would be to install a new hdd , shut down ftp services , then run a mv to the new hdd , then mount the new hdd onto the old folder that had the homes . That way no scripts will fail , the change will be transparent(ish) to the users , and httpd changes are not required.
something like this could do the move / remount on an account by account basis (DONT USE THE BELOW ITS NOT TESTED , IVE JUST BASHED THIS TOGETHER TO GIVE A STARTER ! )
cd /home
ls --format=single-column |awk {'print "mv "$1 " /tmp_home/ ; mount -o bind /tmp_home/"$1" /home/"$1"'} && mount |grep bind |grep home | awk {'print " umount $3 -l"} && mount /dev/newhdd /home
then as each account is moved, the home will be remounted to the new location meaning minimal disruption, then once all the moves / individual bind mounts are done / complete , all those binds are removed , and the new drive mounted to the /home folder.
good luck !