Thunderbolt 3 and USB-C speeds

Since my 2019 iMac has 2 40Gb TB3/USB-C ports…does anybody know if daisy chaining a TB3 OWC raid and a USB-C drive off a single port will slow either drive’s performance down. The RAID only does about 250MB/s according to BlackMagic disk speed test which is well below the 1500MB/s they claim on their web site…and I have no idea why that is. However, 250 MB/s is about 2000 Mb/s so adding the USB-C throughout to that is still well below the TB3 limit…unless the mix of USB and TB on a daisy chain slows things down…in which case I can put a USB-C hub on the second port and separate them.

On a related issue, anybody have an idea why the RAID isn’t faster? Yeah…I know the drives in it are 2.5 inch spinners and hence slower and I know the real world performance is never what the vendor claims…unless they found a specific speed test that is artificially fast…but only getting 1/6 of what the claim is somewhat disappointing…and I know my iMac isn’t a Pro or Mac Pro model… it it just seems like a big gap to me. I’ve got a note into OWC to ask but figured I would get more honest opinions here. The RAID has 4 3TB drives in it and is configured as RAID 5 with the latest version 6.03 of their SoftRAID software and is running under Big Sur.


No, daisy-chaining them won’t slow them down. The TB3 cable that comes with the OWC is likely a 40Gb cable but even if it was a 20Gb cable, the theoretical max for the OWC device is ~12Gb. It’s easy enough to test, drag a really big file from the daisy-chained USB drive to the Mac’s internal drive while you run BlackMagic Disk Speed Test on the OWC.

The magnetic drives are the bottleneck. “Up to 1556MB/s” is just the theoretical maximum throughput for the backplane the drives connect to, basically equivalent to the throughput of two 6Gbps SATA ports (there are four drive ports within but there’s probably some bottleneck in the controller board). At least it’s more honest than advertising the speed of the Thunderbolt 3 interface.

OWC has multiple TB3 RAID products but since you mention 2.5-inch drives, I assume you have the OWC ThunderBay 4 mini. Each drive has a 6Gb SATA connection to the backplane (I’m assuming the backplane isn’t ancient 3Gb SATA) but that’s not a bottleneck, a magnetic drive is much slower than that. Looking at a 3TB 5400rpm 2.5-inch drive spec sheet, it’s maximum transfer rate is 130MBps (~1Gbps); whatever drives you have aren’t going to be dramatically faster.

If the drives were striped (RAID0), the absolute maximum Read speed for the ThunderBay would be each drive speed x 4, 520MBps (~4Gbps), less in reality. RAID5 also stripes data and its Read speed could have the same theoretical maximum. Because writing data to RAID5 requires calculating and writing parity data (work done by SoftRAID using your Mac’s CPU), Write speeds are much slower. My hunch is any Mac new enough to have TB3 ports has enough excess CPU power that using SoftRAID doesn’t meaningfully slow things down, the software is waiting for the drive hardware more than vice versa.

You didn’t say whether the measured 250MBps was the read speed or write speed but even 250MBps Read for RAID5 compared to a theoretical max of 520MBps for RAID0’d magnetic drives doesn’t seem that bad.

The ThunderBay would probably be the bottleneck for four SSDs, any model, 500MBps is an ordinary speed for them (the SATA interface prevents speeds above 750MBps). It would still be slower than the iMac’s internal drive (if it’s all sold state, not Fusion).

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As @cwilcox wrote, there should be enough bandwidth to go around. But make sure you get the devices in the correct order.

You want the chain to be:

Mac --- TB3 RAID --- USB drive

I don’t know if your USB drive has a pass-through port or not (probably not), but if it does, it will only be able to propagate USB data downstream, which will not support a Thunderbolt device (or if it does, the device will behave as a USB device, which will cripple its performance.)

On the other hand, a Thunderbolt device’s pass-through port should have no problem passing either Thunderbolt or USB data downstream, since TB3 is designed to support both technologies.

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Thanks Curtis and David…I did a little more digging on the OWC site and found some BlackMagic and AJA benchmarks in RAID 5 config and they’re bout 300-400 MB/s and I didn’t reboot the iMac and kill everything running so my numbers seem more in line.

I have not purchased the USB-C drive yet…but am researching options…like a moron I only bought 256GB when I got the iMac since my original thought was to use it as a server with the RAID…but then I said to myself why am I using Lightroom on the rMBP when I have the bigger screen back in the office…and for speed purposes you need the Lightroom catalog on the fastest drive you have…and I’m also trying to decide how to handle Lightroom at home and on the road with the laptop for maximum flexibility. My catalog will barely fit on the iMac drive so I’m looking at either a USB-C or TB3 external for it for boot and catalog purposes while still maintaining compatibility and flexibility for on the road Lightroom usage…so I’m trying to gather as many facts as I can and then will do some assorted Lightroom tests on the iMac with the catalog and photos in various purposes to figure out what makes the most sense…most of the portable USBxC drives have only a single port so it will go on the end of the chain anyway.

My measured speeds on the ThunderBay Mini were pretty close…about 250 write and 275 read…I could actually afford to go really high end but the cheap guy in me doesn’t like to do that.

I’m currently leaning toward a dual Lightroom catalog scenario and using export/import to move the travel images into the main catalog and work like heck to get 5(e iMac catalog on an SSD…shoulda been smarter when I bought it but maybe the M1X or M2 or whatever iMac will tempt me to upgrade.

You can only add the USB-C drive to the end of the chain since USB cannot be daisy chained. That’s TB only feature.

Absolutely true, but some USB devices have an integrated hub, which can be used to connect downstream devices. These are somewhat common in USB keyboards, providing 1-2 downstream ports for other peripherals.

I also found at least one USB hard drive enclosure with a built-in hub: NewEgg: USB-C Hub with SATA Hard Drive Enclosure.

If such a device is used, you can only connect USB devices (not Thunderbolt devices) downstream from it.

Absolutely true, but some USB devices have an integrated hub, which can be used to connect downstream devices.

Right, if you use a USB-C hub. But the TB device would still have to be 1st in the chain since it cannot be plugged into a USB-C hub.