Linux mount a new partition on a folder without losing data 

fdisk /dev/sdb
g
n
w
mkfs.ext4 /dev/sdb1
mkdir /var_temp
mount /dev/sdb1 /var_temp
init 1
sudo rsync -av /var /var_temp
umount /var_temp
mount /dev/sdb1 /var
init 5
sudo blkid /dev/sdb1
nano /etc/fstab and add the following line:
/dev/disk/by-uuid/"your-uuid-here" /var ext4 defaults 0 2