Can't mount two external T3 SSDs on M1 MacBook Pro

I’m having a problem with a new M1 MacBook Pro for which, so far, Apple has told me they don’t have a solution. It would be great to know if others are experiencing this problem so we can provide additional feedback to Apple.

I have two Thunderbolt 3 external SSDs manufactured by OtherWorldComputing. Each of the external SSDs works fine when it is the only external device connected to the MacBook Pro. However, when both devices are connected simultaneously only one device receives power and is recognized by the MacBook Pro. I’ve called OtherWorldComputing and they could not suggest a solution. I called AppleCare support and they claim that they have received multiple similar reports and it seems to be a limitation of the new M1 hardware. I was told that it is possible to use two external SSDs simultaneously on the older Intel MacBooks. Currently, Apple does not have a solution to the problem and they could not suggest a workaround. I asked if purchasing a separately powered dock might solve the problem but Apple could not confirm that is a workaround.

Adam Engst told me that he has a new M1 MacBook Air and he has not seen the problem when he connects two external SSDs via USB-C, not Thunderbolt 3. So, the problem may be with external SSDs connected via Thunderbolt 3 but not via USB-C.

It would be great to know if anyone else is experiencing this problem or if they have the equipment can test and let us know their results.


My first test would consist of attaching one of the SSDs to a powered TB3 dock.


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…


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


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.


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.


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 :frowning: 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.


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!


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.