all the site storage is in /var/www
you can simply mount the larger hard drive
as that volume.
stop the ispconfig service(s)
copy the existing contents to a new folder (rename it)
mv /var/www /var/www2
mount the new /var/www (edit your fstab)
('mount -a' would work as well)
copy the old files to the new drive
cp /var/www2/* /var/www/*
now verify files copied
now restart the service
now verify everything is working as it should
check it via your browser,etc.
now remove the old folder