Choosing an external SSD for storing AI models on a Mac
Short answer
Buy a Thunderbolt or USB4 SSD, or a USB4 enclosure with an NVMe drive, for about 3,000 MB/s on any recent Mac, or a Thunderbolt 5 SSD for even faster loads on Macs with Thunderbolt 5 ports. Avoid paying extra for USB 3.2 Gen 2x2 (20 Gbps) drives, because Macs run them at 10 Gbps, about 1,000 MB/s.
Local AI models are big. A 35B model is about 24 GB and a 120B model about 65 GB, and most people end up keeping several. An external SSD is far cheaper than upgrading a Mac's internal storage at purchase, but not every fast-looking drive is fast on a Mac.
Why drive speed matters (a little)
The drive only matters when a model loads. Load time is roughly model size ÷ drive speed:
| Model size | ~1,000 MB/s (USB 10 Gbps) | ~3,000 MB/s (Thunderbolt 3/4, USB4) | ~6,000 MB/s (Thunderbolt 5) |
|---|---|---|---|
| 8 GB | ~8 s | ~3 s | ~1.5 s |
| 24 GB | ~24 s | ~8 s | ~4 s |
| 65 GB | ~65 s | ~22 s | ~11 s |
If you switch models often, a faster drive saves real time. If you load one model in the morning and keep it all day, a 10 Gbps drive is fine.
The USB 3.2 Gen 2x2 trap
Many portable SSDs advertise 2,000 MB/s using USB 3.2 Gen 2x2 (20 Gbps). Macs do not support that mode, so these drives fall back to 10 Gbps and run at about 1,000 MB/s, the same as cheaper drives. Check the connection type, not just the headline speed.
The three sensible choices
| Type | Real speed on a Mac | Best for |
|---|---|---|
| USB 3.2 Gen 2 (10 Gbps) portable SSD | ~1,000 MB/s | Budget, occasional model switching |
| Thunderbolt 3/4 or USB4 SSD, or a USB4 enclosure with an NVMe drive | ~2,800 to 3,800 MB/s | Most people; best value per gigabyte at speed |
| Thunderbolt 5 SSD | ~5,000 to 6,000+ MB/s | Macs with Thunderbolt 5 ports (such as the 2026 Mac Studio), frequent switching between large models |
A USB4 enclosure plus an NVMe SSD you choose yourself is often the best deal: you get Thunderbolt-class speed and can pick capacity freely.
What else to look for
- Capacity: 2 TB at minimum for a model library, 4 TB if you also keep image models.
- Cooling: sustained reads of tens of gigabytes can make small drives throttle. Metal enclosures with heatsinks hold their speed better.
- Cable: use the short cable that came with the drive; a long or cheap cable can drop the connection to a slower mode.
- Format: format the drive as APFS in Disk Utility for the best performance on a Mac.
Moving your models to the drive
- Ollama: quit Ollama, move the
~/.ollama/modelsfolder to the SSD, then set the location withlaunchctl setenv OLLAMA_MODELS /Volumes/YourDrive/ollama-modelsand restart Ollama. - LM Studio: open the My Models tab and change the models directory to a folder on the SSD.
For how much model you can actually run, see how much memory you need.
Frequently asked questions
Can I run models directly from an external SSD?
Yes. The drive's speed only affects how long a model takes to load into memory. Once loaded, the model runs from unified memory at full speed. Point Ollama at the drive with the OLLAMA_MODELS environment variable, or change the models folder in LM Studio's settings.
How big a drive do I need?
Models range from a few gigabytes to over 80 GB each. A 2 TB drive holds a comfortable library of 20 to 40 models; 4 TB suits people who keep several large models and image models.