My first test would consist of attaching one of the SSDs to a powered TB3 dock.
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Simon,
You were right. It was also my first thought to attaching the SSDs to a powered TB3 dock but I was reluctant to try that because it was a substantial investment I would regret if it didn’t work. Well, it worked.
It took me another two calls with Apple Support to confirm that the problem was that the M1 MacBook Pro could not provide enough power through the two T3 ports to power both SSDs. I finally had a scheduled phone call with a specialist Mac Tech Support person. He led me through a series of diagnostics that revealed the problem was that the second SSD could not be mounted because it required too much power. So, on his advice, I purchased a OWC powered T3 dock ($250). Plugged it in and everything is working as expected.
In retrospect it seems that Apple could have designed the M1 MacBook Pro to supply enough power for two external SSDs. However, they may have a good reason for not supporting this. I can’t say. This limitation does change the equation if you are thinking of buying a M1 MacBook Pro with a smaller internal SSD and then buying an external drive with more capacity for your startup disk. You can save money by buying an external SSD with more capacity but if you can’t back it up to another external SSD what is the point? You have to purchase a powered T3 dock in the range $250 - $300 which decreases the cost benefit of going with an external startup SSD in the first place.
I wonder if Apple Marketing figured this out before they finalized the specs on the new M1 MacBook Pro…
-Dom.
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Glad to hear you figured it out. Not the outcome you hoped for I guess, but at least you now know.
I wonder if the 4-port M1 MBP variants (once released) will support more juice.
Does the power limitation extend to any downstream devices added to the port, I wonder? And does the M1 Mini have it too? If so, it’s all the more puzzling given that each port on an M1 Mac has a full 40 gbs bus behind it: Thunderbolt on the M1 Mac mini – When 2 Actually Does Equal 4
David,
Yes, the power limitation extends to another device when added to the second T3 port. I have a bus-powered dock that is not recognized when I plug it in after the M1 MacBook Pro is started from the OWC Envoy Pro EX as the startup drive, even if the dock has nothing else plugged into it!
Thanks for the link to that article. It provided insight into the communication bandwidth on the T3 ports but it didn’t address the power supplied to those ports. I’m not an engineer but the power supplied to those ports seems to be very limited.
-Dom.
Hi Dom, I meant on the same port, downstream in a "daisy-chain” of T3 devices, something I don’t think you tried? Probably a silly question, now I think of it. Unless, of course, more power is supplied to the second port if the laptop is plugged into the wall through the first.
All a bit confusing. I wonder if it’s true for USB devices too? I’ll try it on my M1 Air and let you know.
And the Mini, which is the subject of the OWC article, may be another kettle of poissons entirely—I can see no reason to limit power to their ports. But as you say, they didn’t test that.
David
OK, so it’s not true for USB 3 devices–I have two external bus powered drives plugged into my M1 Air now, and they both mount. Like you I’m not an engineer, but this suggests that the power limitation is for T3 devices only.
The OWC SSD enclosures I bought have hard-wired T3 cables and no other T3 ports, so I can’t daisy chain them 
Also, I’ve tried plugging the power adapter into one of the T3 ports on the MacBook but that only leaves one other T3 port for the external SSD so that is not a solution. Another thing I tried is to plug the power adapter into the T3 port on a bus-powered dock and then plug that dock into one of the T3 ports on the MacBook, then plug the external SSD into the other port on the MacBook, and finally, plug the second external SSD into the dock. That last configuration also does not work. The only way I’ve gotten it to work is to use the externally-powered T3 dock from OWC.
Thanks for your comments and testing.
-Dom.
Interesting result. I don’t have any USB 3 devices to do a similar test on my MacBook Pro. It could be a power limitation for T3 devices or it could be something wrong with my MacBook Pro! Wish I knew.
If anyone else is reading this thread and can test with two T3 devices and two USB 3 devices that would be great!
-Dom.
You’re welcome, Dom. I also look forward to hearing from others.
For the record, my two external drives (in two different bus-powered OWC enclosures, one a cheap 3.0 gen. 1 and one an expensive 3.0 gen. 2) are both EVO 860 SSDs.
I have two Thunderbolt 3 SSDs connected to my M1 Mini, one in each Thunderbolt port. They both show up and work fine using bus power. The SSDs are Sabrent model SB-RKTQ-2TB, and the enclosures are Orico model TOM2T3-G40.
I have a 2020 MBP Intel - same prob. Only one external SSD shows up.
I have a M1 Mac Studio (2022, M1 Max, 32 GB) and it can’t mount two OWC TB3 drives either. Luckily I did have a Thunderbolt dock attached. The dock had a 2TB USB-C SSD in the TB3 port, so I plugged the USB-C drive into one of the Mac Studio’s Thunderbolt ports and the 2nd OWC TB3 drive into the dock, and now everything works.
I notice a similar problem with an SSD and a hard drive on a 2020 Intel. The dock solved the problem but it seems that the ports on a MBP can’t handle the loads. But I’m still a happy Mac user since 1984!
David
Wow, what an absolute arse! Still, amazing what you find on here, and I came here from Google this time.
M2 MacBook Air 13, both TB3 Envoy Expresses (OWC), one Sabrent SSD, one Corsair. The fix is as discussed, use of a CalDigit Element hub. It’s a good thing I had the presence of mind to bring one of those on holiday, otherwise my backup strategy would now be defunct …
It sounds like this bus power limitation still affects the M2, but what about the 4-port MacBook Pro models and the various desktop Macs?
We still wish we had received those. The 14"/16" only came with 3 TB ports. 
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Well, I can only tell you about the M1 and M2 Pro Mini desktops since that’s what I have: both work, supporting two bus-powered SSDs. I ended up using only one because I had enough internal storage space on my M2 Mini to make a data volume that took the place of one of those drives, but while both were connected, they worked correctly.
I do think that this caution about the Airs should be somewhere people can find it though, since it’s clearly catching people out. If you get an Air, get a dock to go along with it.
Right! I was reading back in the thread before they shipped (and I’m looking at Tonya’s old 2016 MacBook Pro with 4 ports) and spaced… 
That’s good to know and makes sense. My guess is that the MacBook Pros will work, but I’d love to have confirmation.
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This is confirmation. 
I’m now the proud owner of my three-port MBP M3 Pro (can’t speak to the two-port). Good news! The two very same bus-powered drives connect and mount correctly even with the system cold-booted and running on battery power. Dock no longer required. Yay!
Just another reason to go with the MBP, I think. The airs are undoubtedly beautiful machines, but it does appear that Pros are for pros.
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