Guide · Mac

Keep two external drives the same

Your photos or footage live on one drive, and a second drive is meant to be its copy. Dragging folders across leaves the copy missing your newest work, and still holding everything you've culled or moved since. Here's how to make the copy match the original every time you plug both in.

First, name the drives differently

Drives bought as a pair usually have the same name, like T7 or Untitled. When two drives share a name, your Mac calls whichever you plug in first T7 and the other T7 1, so the names swap whenever you plug them in the other way round. Stow™ keeps track of which drive is which either way, but you'll want to tell them apart at a glance.

Give each drive a name that says what it is. In Finder, Control-click the drive in the sidebar, choose Rename, and call them something like Archive and Archive Copy.

Setting it up

This uses Stow. Plug in both drives first.

1. Open Stow and click New sync.
2. Under From, choose your work folder on the original drive. Under To, choose (or make) a folder on the copy. Choose folders, not the drives themselves: a drive also holds macOS's own hidden files, which can't be copied.
3. Choose the first option, which reads like Make Photos on Archive Copy match Photos on Archive. New and changed files are copied across, and anything you delete from the original is removed from the copy too.
4. Under When should it run?, choose Whenever Photos on Archive Copy is plugged in. It runs once both drives are connected, whichever goes in last.
5. Read the sentence at the bottom, then click Save and preview results. Stow lists every file it would copy and every file it would remove before anything moves. If the copy has drifted a long way from the original, read the removals carefully. Then click to run it.

The first run can take a while, depending on how far apart the drives are. After that, each run only copies and removes what's changed since the last one. When Stow's menu bar item says Everything up to date, eject both drives.

When you delete something by mistake

A copy that follows the original also follows its mistakes. Delete a folder from the original and the next run removes it from the copy too. Stow doesn't throw it away, though. Anything a run removes or replaces goes into a safety net, a folder called _kept inside the copy, for 30 days. You can put it back from inside Stow.

If a run would remove more than half of what's on the copy, Stow stops before removing anything. That's what you'd see if the original drive came up empty or the wrong folder got chosen. The sync shows Safety stop and how much would have gone. Click Try again to see the list. If you meant it, click Yes, I'm sure. Go ahead.

If the copy should keep everything

Some people want the second drive to hold everything that was ever on the first, culls and all. For that, choose Only ever add instead. Nothing is ever removed from the copy, so it grows by whatever you add and keeps whatever you delete.

If you add work to both drives

If the second drive travels with you and you import onto it on the road, choose Keep both in step instead. When both are plugged in, anything new or changed on either drive is copied to the other. If the same file was changed on both, Stow keeps both versions and puts them in Needs review for you to choose.

Things worth knowing

• A changed file is copied whole. Re-export a 20 GB video and all 20 GB is copied again. Stow doesn't copy only the changed parts of a file.
• Two drives in one room are one disaster away from none. A fire or a theft takes both. Keep a third copy somewhere else, or keep the copy somewhere else between runs.
• Check that it opens. Every so often, open a few photos and clips straight from the copy. A backup you've never opened is a guess.