Get a drive that's at least 1-2TB in space, preferably 4TB.

But if you're considering getting a new drive for this endeavor, here's what I suggest: What next? If you already have an external drive, you can always use it for storing your Photos library (and save on cash). Okay, so you've decided to move your Photos library over to an external drive.

If you use an automated backup service for your computer, you'll have to set up a second set of rules for backing up your hard drive.

You don't want to spend the money on an external drive.

Unless you purchase a speedy drive, working externally is almost always slower than working on your default hard drive.

You don't want to accidentally create duplicate libraries that can't be connected to iCloud Photo Library (because your offsite library is connected).

You use a laptop frequently and can't afford an SSD, and you don't want to risk breaking your disc-based hard drive by constantly moving it.

You don't want to have to worry about always having your external drive connected to view and edit your images.

You don't have a big enough library to warrant moving it off your Mac.

Here are some reasons you might not want to use an external drive: That said, there are some downsides, too.

If you're working with space constraints on your primary computer, it's one of the easiest ways to back up your full iCloud Photo Library.

You share a computer with other users and you have limited drive space.

Your library isn't huge, but you'd prefer to save space on your Mac for other files.

You frequently swap computers and want to work on your images from any Mac.

You have a giant Photos library (and, if you use iCloud Photo Library, you want to ensure you have a secondary backup of everything in that library).

There are a number of reasons why an external drive might make sense when you're working with Photos for Mac: Why you should (and shouldn't) use an external drive with Photos for Mac

How to create a new Photos library on your external drive.How to move your current Photos library to an external drive.Best external drives for storing photos.Why you should (and shouldn't) use an external drive with Photos for Mac.Here's how to do it, and some reasons why you should - and shouldn't! - consider it for your needs.

There is an alternative: Moving your Photos library to an external drive (or creating an entirely separate library and syncing it with iCloud).

Video using apple photos