# list existing block devices
fdisk -l
--> /dev/vdb
# partition the new block device
fdisk /dev/vdb
n
p
<enter>
<enter>
w
# format the new partition
mkfs.ext4 /dev/vdb1
# disable automatic fs-check every X days/mounts
tune2fs -c0 -i0 /dev/vdb1
# stop services potentially accessing /var/qmail
service postfix stop
service dovecot stop
# rename old qmail directory and create new one
mv /var/qmail /var/qmail_old
mkdir /var/qmail
# get UUID of the new partition
blkid
--> e725875b-0153-49c6-8e9b-132b0acf0b30
# edit fstab to make system automatically mount the new partition/block device (on boot)
vi /etc/fstab
--> added "UUID=e725875b-0153-49c6-8e9b-132b0acf0b30 /var/qmail ext4 errors=remount-ro 0 2"
# mount the new partition
mount -a
# move content from old qmail directory to new parition/block device
mv /var/qmail_old/* /var/qmail/
rmdir /var/qmail_old
# starting the stopped services again
service postfix start
service dovecot start