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macrumors newbie
Original poster
Mar 16, 2014
Ok, I could use some help, even if it's to tell me my idea is doomed. I bought my iMac used and the genius who purchased it new loaded it with 24gb of ram, but no ssd, so it was slow as hell. I bought an external ssd, and have been booting from it and it was working great.

Of course, there is something wrong with me and I want it to to be faster. I put together an external NVME drive that can run off USB-A, USB-C or Thunderbolt 3. External NVME enclosures that run off Thunderbolt 2 were more expensive and basically obsolete. I have been running it over my USB 3.0 port and while it was faster, I wanted to try and run it through my Thunderbolt 2 port for more speed.

I purchased the Apple Thunderbolt 2 to Thunderbolt 3 adapter, along with a powered Elgato Thunderbolt 3 dock.

The problem is I cannot boot from the drive over Thunderbolt 3. When iMac is running, it can access the Thunderbolt 3 drive through the Thunderbolt 3 dock and Thunderbolt 2 to Thunderbolt 3 adapter, BlackMagic Speed Test shows the speed bump I was hoping for (double). But when I select it as a startup disk it doesn't boot, and after holding option during startup, the disk is not displayed as an option in the boot menu. If I plug the same drive into one of the USB ports on the dock it is then bootable, but lacks any speed increase so there's no point. If anyone out there has any experience or insight it would be greatly appreciated.

Thank you,


macrumors Penryn
Feb 20, 2009
If all you have is USB3 and thunderbolt2, my recommendation is to boot from a USB3 SSD.
Or, use a tbolt2 external drive with an SSD inside.

You can -try- getting a boot drive to work from a tbolt3 dock connected through the adapter, but I'm guessing that it's going to be "problematical" to get it booting reliably. I could be wrong.

I doubt that you're going to get an SSD to run "faster" through a tbolt3 dock that's connected to a tbolt2 port.
Probably no faster than the USB3 SSD. Again, I could be wrong.


macrumors newbie
Original poster
Mar 16, 2014
As stated in the OP I get double the speed when I run it through the dock. But you’re right, so far booting has been problematic.

I can boot from the dock and adapter through one of its USB ports with the same drive, just not the Thunderbolt port on the dock.


macrumors member
Nov 22, 2009
I have the exact same problem.

15" MBP with Thunderbolt 2 ports. Connected the port to a Thunderbolt 2 cable attached to an Apple TB2 to TB3 adaptor, plugged in to a powered Elgato Thunderbolt 3 dock with a TB3 cable attached to my new 2 terabyte PCIE NVME M2 SSD.

I cannot boot from the external drive using Thunderbolt, but can boot using a USB connector which is about half the speed of Thunderbolt. (I tried both the Option key trick and changing the Startup Disk in System Preferences) I can see the new SSD and access it if connected with TB though.

Any ideas? Is this an Elgato problem or a Thunderbolt issue?

3SQ Machine

macrumors regular
Dec 8, 2019
I researched this issue a bit when I was trying to do the same on my older iMac. You didn't tell us the year, but I'm guessing it's got to be a '14 or '15 because I believe those are the only two TB2 years (earlier had TB1 and '17-'19 TB3).

This article (slightly dated) will help you. At least the author shows some of the challenges and what was done to solve them. YMMV.

I settled on NOT going down this road and instead used USB3 capped at 5gb/s SATA. Beyond the power issue with TB3 to TB2 the old iMacs in general were finicky with NVME. You might have some luck looking for threads of people who installed (internal) NVME into their older iMacs or MBs for ways they made it "just work." My own research brought up enough question marks to make me unconvinced of NVME via TB2 for its reliability & compatibility as a system drive. As an aside, you might want to see if you can find a native TB2 enclosure compatible with NVME but I only recalled finding them in SATA, which wasn't worth the insane price premium. I think some are mentioned in the article. Good luck.


macrumors regular
Oct 23, 2009
I have a Glyph Thunderbolt 3 dock which has an inbuilt M.2 NVMe slot with a 1TB WD Black in it. I use it with my 27" iMac 2019 i5 3.7, to test run Catalina.
I have swapped it over to my 15" 2012 MacBook Pro (non-retina), and the Glyph dock (connected via an Apple TB3-TB2 adapter) boots the MBP into Catalina with no problems.

The MBP is only TB1, and the WD Black benchmarks at 826MB/s read/868MB/s write with Blackmagic Disk Test - which is what I would expect from TB1.
I would presume TB2 would give the full speed of the WD SSD (in the Glyph dock), as the dock only runs at PCIe 3 x 2 speeds (about the same as TB2), with the rest of the TB3 bandwidth being used by the other 7 ports.

So booting NVMe over TB2 is possible with the right kit.
(The Glyph dock only takes single-sided NVMe SSD's - Samsung EVO, WD Black and Intel 660p/760p are the ones the manufacturer recommends).
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