Thunderbolt is supposed to allow 6 devices per port/bus daisy-chained*, but users with more than two devices daisy-chained almost certainly have run into the dirty little secret of Thunderbolt daisy-chaining that neither Apple nor Intel will discuss: sporadic random disconnects.
Ever have macOS tell you that a device has disconnected unexpectedly (that you should eject it first?). If it’s on Thunderbolt and you didn’t physically do something, that’s a disconnect bug.
I’ve had severe problems with disconnects over the years, and cable swapping with new cables has never fixed anything.
Low-level usage doesn’t provoke disconnects very often, but put a heavy I/O load on that Thunderbolt bus and the odds of problems spike dramatically. This is a huge headache if you are trying to let an overnight backup complete, or you are validating data integrity, or need to process a ton of video for the next day’s work.
Three devices can be stable... until it isn’t. Four devices you’re just asking for trouble, five or six (the max) and you might as well keep keep a slab of concrete handy just to bang your head against so the disconnects don’t feel so bad.
In my experience, daisy-chaining three or more devices is just asking for trouble.
* Daisy-chaining refers to connecting devices one after the other; if one device goes wonky, then all the other devices after it are affected, and maybe the ones before it as well.
More ports with total reliability: the OWC Thunderbolt Hub
Get OWC Thunderbolt Hub at macsales.com.
Got an iMac 5K with only two Thunderbolt ports? Add the OWC Thunderbolt Hub and you have 3 Thunderbolt ports plus a USB 3.2 10Gbps port. Plug in two hubs and you have 6 Thunderbolt ports, plus two USB 3.2 10 Gbps ports.
The OWC Thunderbolt Hub has two key benefits:
- Helps eliminate daisy-chaining and all its disconnect headaches.
- Turns a single Thunderbolt port into three, and adds a USB 3.2 10 Gbps port.
Plug the Hub into the Mac, and then plug in your devices. Those ports all share the bandwidth, but that’s not an issue with even 14 hard drives as in my test. Besides, the same limitation applies were daisy-chaining used.
With the Hub, the four devices now each have their own separate chain:
Mac => Hub => device1
Mac => Hub => device2
Mac => Hub => device3
Mac => Hub => device4
Without the Hub, it would be (in my years of experience) a highly unreliable daisy chain:
Mac => device1 => device2 => device3 => device4 <---- 4-device chain = trouble
OWC Thunderbolt Hub In use
My goal was to do a major reorganization of all my data, which meant bringing all backups to date from 32TB of SSD storage consisting of: 16TB OWC Mercury Accelsior 4M2, 8TB OWC Thunderblade, 8TB Apple internal SSD. Not all full, total data of about 15TB, spread across the SSDs.
All of this needed to be backed up onto the external drives, which was complicated by the fact that the 14TB drives could not hold it all; I had to split the data across a mix of 14TB drives, 8TB drives, and some 18TB drives and 28TB striped pairs of 14TB drives.
I happened to have the perfect real-world stress test for the OWC Thunderbolt Hub: a major reorganization of my backups which in total were some months out of date because I knew it would take a whole weekend to get the job done and I had procrastinated (I did have adequate first-level backups).
Starting Friday night, I eliminated all daisy chaining from my system, connecting three OWC Thunderbay 4 units to the OWC Thunderbolt Hub, along with an OWC Mercury Elite Pro Dual (36TB) via the 10 Gpbs USB-A port on the Hub. That’s 14 drives in 4 devices into the Hub. I also directly connected an OWC Thunderbay 6 to another port on the 2019 Mac Pro. Together with the OWC Thunderblade also directly connected to the Mac Pro.
I set Carbon Copy Cloner to work and wondered whether I’d wake up in the morning and see 8 or 10 disconnected volume notifications. To my surprise and delight, things were still running flawlessly on Saturday morning.
Never before in my use of Thunderbolt have I ever been able to move around a hundred terabytes or so of data with ZERO problems.
Yet that is exactly what I experienced from Friday night to Sunday night: zero disconnects, zero glitches, perfect function. And that’s why I say “unprecedented”—heretofore I have never been able to reliably do this amount of data transfer withut disconnect problems.
There is one downside—Thunderbolt hubbing support requires macOS Big Sur. But this is such a big relief in reliability that it might actually be worth it.
Should you need the same hub capabilities along with some additional ports and/or gigabit ethernet and/or 3.5mm audio out and/or an SD card reader, the OWC Thunderbolt Dock supplies those ports (see image below). If you don’t need those extra capabilities but do want more Thunderbolt and USB ports, then two Hubs is the better choice, for only a little more money.