External SSD or hard drive for your Mac: pick by what you are storing, not by price alone
Hard drives are cheaper per terabyte, SSDs are faster and tougher. Which to buy for Time Machine, photo archives, video editing or running apps.
The choice is simpler than the shelf of drives makes it look. Decide what goes on the drive, and the type follows.
| Use | Choose | Why |
|---|---|---|
| Time Machine backups | Hard drive | Large and cheap, speed rarely matters overnight |
| Archive of old photos and projects | Hard drive | Written once, read rarely |
| Photos library you open daily | SSD | A hard drive makes browsing sluggish |
| Editing video from the drive | SSD | Needs sustained speed |
| Running apps or games from it | SSD | Launch times on a hard drive are painful |
| Carried in a bag | SSD | No moving parts to damage when dropped |
Speeds you can expect
| Drive | Typical speed |
|---|---|
| Portable hard drive | About 100 to 150 MB per second |
| USB SSD, 10 Gbps | About 800 to 1,000 MB per second |
| Thunderbolt or USB4 SSD | About 2,000 to 3,000 MB per second or more |
Your Mac's own SSD is faster than any of these, which is why moving everything off it is not the goal. Move what is large and rarely used.
Test what you have
# Write a 2 GB test file and time it (replace the drive name)
time dd if=/dev/zero of=/Volumes/YourDrive/test.bin bs=1m count=2000
rm /Volumes/YourDrive/test.binDivide 2,000 MB by the seconds reported for a rough write speed.
One more thing: cables and ports
A fast SSD on a slow cable runs at the cable's speed. Use the cable that came with the drive, and plug it directly into the Mac rather than through a cheap hub.
Setting it up: moving files to an external drive and formatting for Mac and Windows. For developers, external SSD for a developer Mac.
Common questions
Is an SSD or HDD better for Time Machine?
A hard drive is usually fine and much cheaper per terabyte. Backups run in the background, so speed rarely matters.
Should I keep my Photos library on an external hard drive?
It works, but browsing is slow. An SSD makes a Photos library on an external drive feel like it is on the Mac.
Why is my external SSD slow on my Mac?
Usually the cable or a hub. Use the cable supplied with the drive and plug it directly into the Mac.