Setup external hard drive

You should choose an external hard drive that will spin down automatically after idle(no I/O activity) for some time. And in order to let it idle, you need to dismount the drive; that leads to the use of autofs.

Add this to /etc/auto.master:

# dismount after no file access for 120 seconds:
/mnt /etc/auto.exthd --timeout=120

Create file etc/auto.exthd with this:

exthd -fstype=auto,rw :/dev/sda1

Restart autofs to take effect.

After that, whenever you access /mnt/exthd, the partition /dev/sda1 will be auto mounted.