It completely depends on what your preference is for storage and how you want to run the drive.

Here are the options:

If you want to run everything from the external HD, yes. You will need to have it plugged in. If you wish to run the main program on the internal HD but keep all the bigger files on the external HD and set up customer folders to that external HD, yes; it will need to be plugged in. If you put everything on the internal HD, then no. You don't need to have it plugged in.

We hope this helps!

Cheers,
Ember