Hi alanchanrs
I have a 'G technology Drive' its just plug in and play, its already formatted for Mac, its 7200rpm.
Mine connects by USB, when you connect it a little icon comes on the screen to show you its connected.
Anyway, you don't need do anything, you open Logic and carry on as usual, then at the end of the session click FILE, at the top in Logic, SAVE, you will be given the option to save the project/session to the External drive, just click it, your session will be saved to the external drive.
Then, when you have finished working, you drag the External drive icon to the trash, (this is called dismounting) the icon will disappear, then you can turn the external drive off.
Next time you want to work on that particular session, just open it from the drive, and the same with all future Logic projects, you can save to the drive, all the files and stuff.
Your computer system drives just runs Logic for you, the files etc go the external drive.
Ideally you should have a further external drive to make back up copies of external drive number 1!
I have 1 external drive for all my Logic projects, then I make a back up to another identical external drive.
I also have yet another drive for Time Machine back up, but don't worry about that now!
I or whoever can advise on here later if you wish to know.