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Life in Thunderbolt 3 Hell: a Single Thunderbolt 3 Bus Plus Thunderbolt Cabling/Ports Are a Severe Headache

Due to getting low on primary disk storage, I spent most of the last week rejiggering my backups.

I found (as I have before) that Thunderbolt 3 is so damned flaky that one wonders how it got out of the lab—and I say that having tried all sorts of things including new cables.

When there is only one Thunderbolt 3 bus (e.g., 2019 iMac 5K), Thunderbolt 3 is pretty much a nightmare technology if trying to use more than three Thunderbolt three devices. I am looking forward to the 2019 Mac Pro for that reason, which has at least two Thunderbolt 3 busses (more if certain cards are installed).

Thunderbolt 3 ≠ 6 devices

Thunderbolt 3 is supposed to accept up to six devices on the bus. I had trouble with only five, as well as trouble with only four devices.

Attempting to use a 5th device would typically fail out, with one of them not mounting the drives at all. The behavior is (1) drives spin up, (2) drives clunk off, (3) repeat ad nauseum. This happened with different brand storage devices. And it’s not just my machine; I’ve heard from the developer of SoftRAID that he can rarely succeed with more than 4 devices. And I see the same issue on the 2019 Mac mini if I try to use four or more devices on one bus (but it has two busses which makes for an easy work-around).

In the end, the only way I could complete my backups was to disconnect one of my three “always there” devices so that I could attach one backup drive, for a total of three Thunderbolt 3 devices. Later, I got it to work again with four devices.

When done with the primary backup, I moved that backup to the 2019 Mac mini, so I could clone it to other backups. I had no other viable choice given the flakiness issues.

Thunderbolt 3 cable length limitations

Thunderbolt 3 sucks in that a Thunderbolt 3 cable cannot be more than 2m/6ft long, which means that with two machines about 4 feet apart, cables just don’t reach from desktop to under the desk. So I had to physically move the backup devices to the other machine, rather than just routing a cable and plugging it in.

Thunderbolt 3 port flakiness

Just like Thunderbolt 2, Thunderbolt 3 ports/cables are so badly designed that doing little more than breathing on them can cause a disconnect. Try recabling without shutting down and you’ll see what I mean. The OWC ClingOn helps tremendously to avoid disconnects, but it pretty much locks the cable in place (good), which is troublesome when things need to be recabled, as in my backup situation.

Thunderbolt 3 disconnects can corrupt a file system, as I found out. It meant redoing two backups all over again—a 24 hour process.

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