external HD

There's no reason to leave your external drive on all the time, but much depends on how you use your machine.

For example, I have an iMac and an external drive. I use the external drive exclusively for backup. It seems you're using it for archiving old files, and possibly backup. In that case, there's no reason to power up your back ups all the time; just sync the drives when you need to. However, if you use the external drives continually, you may have to keep them powered continually.

Post with additional questions!

The big benefit of an external (preferably portable) HD is that it can be protection against the potential problems of Machine theft or major breakdown and/or natural events. As such it is best topped up every so often and then located separately in another area.