Guru required: Why does TB connected external SSD show slow write speed on 2019 iMac

IF speed is critical to you, then the answer is really simple. Buy quality components that support the speeds you are after. As with all things "computer" the bottleneck of any system will be the weakest component. For an external SSD that could be any number of items including:

• The SSD itself
• The SSD's enclosure
• The cable used to connect the SSD
• The port used on the computer

High speeds can be achieved but not by using "cheapie" parts. If you want the best speeds, you need to be using OWC (www.macsales.com) components. If you cannot match them yourself then call OWC and speak to a consultant that can help you match the components. you may not like the price (quality does not come cheap) but if performance is your goal you need not go any further.

A 2019 iMac's T3 ports support USB 3.2 Gen 2 (originally named USB 3.1 Gen 2, or Superspeed+ USB). It is rated by the USB consortium or whatever they call themselves for a 10Gbps transfer speed and a theoretical throughput of 1250 MBps (1250MBps x 8 b/B = 10Gbps).

1457 MBps is already exceeding that speed the even higher read speed is probably due to the drive ability to transfer larger chunks into a direct memory access buffer. It's free chicken, basically, and an unexpected bonus.

The limiting factor is the port on the 2019 iMac being only Thunderbolt 3/USB3.2, not USB4.

The OWC chart posted is not showing any Thunderbolt rates.....everything shown is regarding USB transfer rates as noted in the 2nd column. No where in the chart posted does it actually show Thunderbolt transfer rates.

Are any devices connected to the second USB-C port on the 2019 iMac when you are running the speed tests? If so, then that device will affect the speed tests since the non-Pro iMac 2019 only has a single Thunderbolt 3 bus.....so that means both USB-C ports share the total 40Gb/s bandwidth. The 40Gb/s bandwidth is a total in both directions on both USB-C ports. I believe I recall reading somewhere that the bandwidth is not shared equally like you may think so you may get a lopsided limitations as well.

Plus I learned long ago that Macs are very picky about the devices that can be used.

FYI, I believe I read a different post on this forum regarding a Zike device. The user contacted Zike support and found out there was a firmware update for the Zike device which fixed their Apple compatibility issue (I don't recall the specific issue now). Have you checked the Zike support site to see if any firmware updates are available, or to ask them these questions?

Also, when posting read & write speeds, please post the read & writes in the same order. Reversing the second set makes it hard to keep things straight.

No, I have no opinions on that. Generally I say to go with a brand you trust and know to be reliable and perform within expected parameters. If you get lucky and something does better than expected, bonus.

But I rarely if ever support chasing ever-increasing performance boosts - it's a game of diminishing returns and the actual real-world impacts are so close to zero to make the effort worthless. Why spend thousands of dollars and hundreds of hours getting your network latency below 4ms, for some theoretical advantage over a 10ms latency network that can't possible affect your gaming experience (channelling shadows a real-life story I know of, if you couldn't tell).

So, could a WD Black SN850X outperform a Z666 on your system...maybe. Or maybe not - maybe it's a fluke unique to your computer's logic board, not the drive attached to it. Is it really worth spending money on to find out and maybe get a marginal advantage? That's your call.

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