Tracy's Blog

Tips for Mac users

M1 Mac external drive write is WAY TOO SLOW


So, you’ve got a new M1 Mac, and a nice fast NMVe in an external Thunderbolt 4 enclosure, and you’re only getting 640MB/s write speed? WAAAAAY TOOOO SLOOOOOW!!!

That was the issue that just took me three days and $220 to resolve.

(Turns out I didn’t need to spend the $220, either…)

The issue:

I put a 1TB NMVe drive into a Sabrent Thunderbolt enclosure, with the intent of using it with my M1 MacBook Air laptop (and soon to be Mac Studio.)

I ran Sensei on it to check to the speed, and it was -terrible- : 640 MBs write. (The read was a more reasonable 1300.)

I went through every test I could think of, including using a different drive; pluggin it into the other TB port; plugging it into the Caldigit TS3 hub; plugging it into my M1 MacMini. All the write speeds were pretty much the same: half the speed it should be.

OK…. sigh… Must be that it’s a well-used drive, which I pulled from my main machine about a year ago.

So, I hop on Amazon and buy two new WD 1tb drives, for $220. (I’m still waiting for them, but read on…)

Again, using Sensei (highly recommended, BTW – shows more useful info than anything else I’ve used) I see that the drive report shows:

80% life remaining
Data read: 98TB
data write: 75TB
read commands: 6.5 billion
write commands 1.6B
Hours 11,800

(the really high numbers are because this was the boot drive, with Apple incessant writing to it)

OK: that’s a lot… so I pulled another 1tb drive with these specs:

100% life remaining
Data read: 3.3TB
data write: 760 MB
read commands: 44 Million
write commands 8 Million
Hours 15,000

(More time in use, but about 1/10th the wear and tear.)

and threw it into the enclosure.

Exact same results! Grrrr…

I call Sabrent, thinking at this point, it had to be the enclosure. Tech support was great, and we tried all kinds of things, ending up with doing a secure(!) erase (which took 4 hours) and reformat. That bumped the speed up to 720.

So, by now I’ve run the tests on two different M1 machines, with two different NMVe drives in the enclosure, with 100% consistent results.

Then, 30 minutes ago, and by pure happenstance, I ran the test with my Caldigit hub in one port and the enclosure in the other mac port…

… and got nearly 1200 for my write speed!


Turns out that the TB ports respond weirdly: I found this on the OWC website…

“the M1 CPU only supports one video output signal to its TB4 ports. When a Thunderbolt display is connected to an M1 Mac’s TB4 port, the other Thunderbolt 4 port on the Mac does not need to support video output. It can then deliver the full PCIe data bandwidth potential to an attached external drive. “

Get Up to 67% Faster External Drive Performance With Your M1 Mac!

I don’t run any video off my TB ports on either machine, BUT… the Caldigit hub -offers- a video connection via HDMI. It’s not connected but apparently the M1 sees it as a connected video monitor, and the explanation above works!


And now you know!

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