advice running seagate 3tb on imac

What do you want to use it for?

Is this to be an external backup drive? For Time Machine or a clone?

In either case, you need to format it Mac OS Extended (Journaled) and make sure it's GUID partition scheme. That is all that is required for a "setup".

So mostly a drag 'n drop? Or will you be putting your libaries on it (meaning they won't reside on your internal at all)?

Again, formatting is the only thing required; during that process, any pre-installed software from the mfr. will be deleted. That is fine as you don't need it.

You could partition the external drive; however, if all of it is going to be used for video, there doesn't seem to be any reason to do so. Do remember that if that drive will be the only place for your stuff, then you will not have a backup. All drives fail sooner or later......

First, the drive needs to be formatted: use Disk Utility (Applications > Utilities), highlight the external, click on Partition tab, (**next step: see below), make sure it is formatted as Mac OS Extended (Journaked) and check under Options that it says GUID partition scheme. Doing that will automatically erase the entire drive and format it correctly.

**: As for partitions: if you just want one, then choose 1 partition. However, you said something about Time Machine. If you don't want it, make sure it is turned off (Systam Preferences) and if it does ask, simply say no thanks (I have mine turned off and it always asks - a bit annoying). In any case, if you do want it, then you will need to create more than one partition as it needs to be on its own partition. Just choose 2 partitions - name them (TM and video?? or whatever) and then proceed as above. You will then get two icons on your desktop instead of just one. You can drag 'n drop to your Video partition and launch Time Machine to do its thing.