USB-C cables have vastly different capabilities

USB-C is a wonderful standard. Apart from the ability to plug it in either way up, it supports Power Delivery and much higher speeds.  I've run in to some mystifying scenarios where a cable would charge but not carry data, would carry data but not charge, and lower than expected data throughput.

I'm a registered Apple developer and was keen to try out the macOS 15 Sequoia beta but I'm not an idiot so decided to install it on an external disk on my Mac mini.

The steps were:

• Used Disk Utility to format a Samsung T5 drive as APFS
• Downloaded the Sonoma macOS installer via the App Store
• Installed Sonoma macOS 14 on the external drive
• Booted from the external drive and enabled Beta Updates in the Software Update settings and chose macOS 14
• When all that was done I booted from the external drive.

Sequoia seems quite stable and I've been able to build my applications with the Xcode beta.

But.... it did seem rather slow! I used the Blackmagic disk speed test to measure the drive and got Write: 38.9 MB/s & Read: 39.3 MB/s. 

The USB3.1 port is capable of 10Gb/s or 1,250MB/s so something is up.

Changing USB-C cables to one marked as 10Gbp/s I re-measured and got Write: 263.6MB/s & Read: 344.9MB/s. Almost 10x improvement but well short of what's possible.

Today, I bought a 1TB Western Digital drive. Blackmagic gave me Write: 804.1 MB/s & Read: 906.8 MB/s.

The Mac mini has two Thunderbolt/USB4 ports that are capable of up to 40Gb/s so I have ordered a Thunderbolt enclosure.

Oh, and just a note that the speed of the internal SSD on a mac is pretty amazing. Blackmagic measures the internal SSD as Write: 3049.6 MB/s & Read: 2856.4 MB/s. I guess that's why I noticed the slow speed.

Summary: If things seems slow, try another USB-C cable. (I wish they had a standard for markings on the cables with their capabilities).

Update: faster external disk

Wanting to get closer to the speed my interfaces are capable of I ordered an NVMe SSD stick and case from AliExpress. Plugged in to the USB-C socket on the back of the Mac mini I get the same speed plugged in to the USB-C hub outlets on my monitor so I'm using that. Speed is pretty good. Close to 1GB/s.

The enclosures are pretty nice. Mine came with a heatsink, sticky pad and even the screwdriver needed to open it. Not as small or neat as the Samsung T5 but five times faster.