Most likely you have been buying hybrid USB4/TB4/USB-C NVMe enclosures that support Thunderbolt — but are not true Thunderbolt-only devices.
Those “multi-standard” enclosures contain both:
- a Thunderbolt controller
- a USB fallback controller
…and older Macs will sometimes negotiate USB instead of Thunderbolt depending on firmware, cable, controller type, or compatibility quirks.
The result ,as you have seen, is that they show up a USB 3.1.
Ok, so do you have any real choices for "real" Thunderbolt 3 enclosures? Yes!
Look for:
- Intel-certified Thunderbolt 3 NVMe enclosure
- Preferably TB3-native (not USB4-first or hybrid USB/TB designs)
Here are a few I suggest that you consider:
One thing to consider if you expect to have sustained writes/video editing workloads is to look for enclosure that provide active cooling. One that I'm aware of is the Fledging Shell Thunder.
For reference, the 2017 iMac are equipped with early Thunderbolt 3 (Alpine Ridge) controllers:
- More picky about controller firmware
- Some Titan Ridge / USB4 hybrid enclosures downgrade to USB mode
- Certified TB3 devices tend to work consistently because they force TB tunneling.
Finally, be sure that you use real Thunderbolt cables, not USB-C — this alone can cause USB fallback.
If you tell me:
- whether you need portable or desktop-style
- single NVMe vs RAID/dual NVMe
- the SSD model (Samsung 990 Pro, WD SN850X, etc.) you will be using with this enclosure ...
…I’ll give you my suggestion for a specific enclosure that I would deploy myself for a 2017 iMac video editing workflow.
