best external drive

There are many fine external drives available at moderate costs. Many come with their own management software which should be deleted and not used. It often causes problems. A USB connected drive will be less expensive than a Thunderbolt drive is and should serve your needs. The drive may have to be formatted prior to using it.

If you plan on moving your Photos Library off to an external drive, then I suggest having more than one external drive. One for your Photos Library and one to backup both the iMac and the Photos Library on the external drive.

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