mkfs.ext4 command not found in Debian (Jessie)
Do you have
/sbin in your path?
Most likely you are trying to run
mkfs.ext4 as a normal user.
Unless you've added it yourself (e.g. in
/etc/profile etc), root has
$PATH, but normal users don't by default.
Try running it from a root shell (e.g. after
sudo -i) or as:
sudo mkfs.ext4 -L hdd_misha /dev/sdb1
BTW, normal users usually don't have the necessary permissions to use
mkfsto format a partition (although they can format a disk-image file that they own - e.g. for use with
FUSE or in a VM with, say, VirtualBox).
Formatting a partition requires root privs unless someone has seriously messed up the block device permissions in