Thunderbolt 2 Drive

I’m thinking adding a Thunderbolt drive to my ancient iMac (Retina 5K, 27-inch, Late 2015) for Time Machine backups.

This iMac’s four USB ports support 5 Gbps, which is really slow for moving gigabytes —much less terabytes—of data. It also has two Thunderbolt 2 ports at 20 Gbps.

I presume that new Thunderbolt 3 and 4 SATA drive enclosures are backwards compatible with Thunderbolt 2. But I see that Apple sells a Thunderbolt converter adapter so maybe I need one to connect a current Thunderbolt drive to my ancient iMac ? :man_shrugging:t2:.

Any advice?

UPDATE December 1, 2023 2:08 PM

Apparently, Thunderbolt 1 and 2 use a Mini DisplayPort connector whereas Thunderbolt 3, 4, and 5 use USB-C so I definitely need an adapter.

Is Apple’s the best (or only) one to get?

I have an Envoy Express from OWC ( in which I installed their 1.0TB OWC Aura Ultra IV (; these also come pre-equipped up to 8TB. (I did this so I could make bootable backups on one partition, and Windows on the other; I know, that’s apparently not your need.) It’s amazingly fast, seems to be as fast as the internal drive on my M2 MBAir. Might work for you.

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One caveat - The Apple Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter does not supply power:

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TM is i/o throttled. It’s highly unlikely you’ll ever see it do >200 MB/s even on the fastest external storage. (And some more on that here.)

So unless you want TB over USB because you want faster access when copying back from those TM backups, you might as well just go with the slower but much cheaper and larger selection of 5 Gbps USB 3 stuff. Macs push 400+ MB/s over USB3 which still easily outperforms the throttled TM writes.

Me, personally, these days I usually opt for inexpensive “slow” (<1 GB/s) NVMe SSDs for TM to APFS. There’s a wide selection of quality flash there. These SSDs are much more convenient than HDDs (reliability, size, noise, power) and since you don’t care about getting very fast SSDs due to TM throttling, they are not expensive or require interfacing through more expensive TB either. NVMe to 3.1 gen 2 bridges are very inexpensive and zero fuss.

I’ve also never been a big fan of choosing very large disks for TM. I prefer getting a medium size drive where you always get the very best GB per $ and then swapping out for a new drive once that drive reaches ~80%. Your TM disk will always be fairly young and reliable while the old disk is going nowhere. You can always reach back to it should you need something from older TM archives.

But outside of TM, if you want performance, get a good TB3/4 sleeve and a high end NVMe SSD for the best external drive performance at still relatively low cost. Here are a couple simple examples.

Random thought, another take on you problem:

Part of you problem may be related to how large your files are. If they happen to be videos, you can compress them. I use HandBrake, which can often reduce video files to no more than 25% of their original size. They’re then perhaps not as detailed as the originals, but on a computer screen the difference is hardly noticeable.

Thank you very much for posting this information about Time Machine I/O throttling. (I follow The Eclectic Light Company but obviously forgot about this article.)

I don’t care as much about the time that how much time that Time Machine takes to back up a disk as a do about how much time it takes to do a restore.

Do you know whether Time Machine restores are also throttled.

And do you know anything about throttling Carbon Copy Cloner (“CCC”) backups or restores? Yesterday I restored a 3.9 TB CCC backup from an external hard drive with a 5 Gbps connection to an internal SATA SSD and was surprised that it took 10 hours running by itself overnight.

Thank you.

I have a 2017 iMac with UB C thunderbolt ports. My external drives are in an Akitio Thunderbolt enclosure with DisplayPort connector. I use the Apple adapter. I have had no problems at all with this setup. It has been very reliable, but your hopes for much speedier transfers may be a bit unrealistic. While I have found the Thunderbolt to be a faster reliable connection, the bottleneck is your SATA 3 disc. Google SATA specs and see what you are up against.

That’s interesting. Large amounts I usually restore very rarely, almost only in the rare event of catastrophic failure (during migration, the source Mac is usually still around and well so I get it straight from there rather than from the TM volume). Most of my restores from TM are individual files/folders (basically getting back an old version). Not a whole lot of data to transfer so speed is of little importance. But obviously, everybody’s use is different.

This i/o throttling is not a global thing (see Howard’s explanation of the 5 different policies and their applications). But it is well documented for TM backup (as a classic example of a low-priority background task [and this incidentally is an assumption that does fall apart for the initial large backup]). But of course if you just do a simple Finder copy (like eg. when you’d copy back an old file/folder from a TM volume), it does not apply. I can easily see my Finder copy @ 800 MB/s from/to the same external drive to which my TM otherwise never writes at more than 200 MB/s.

If your main concern is getting large speed when copying back then obviously the 2.8 GB/s that you could achieve (max) with TB3/4 is attractive compared to the very best USB-C 3.2 gen 2 rates observed on Macs. (~1 GB/s). And if you truly need/want that speed, the ~$100 difference in cost for the TB over the USB bridge is likely negligible.

I’m afraid I do not since I haven’t used CCC in years. However, Bombich is known to offer lots of good documentation and being very responsive. I’d check with him which policy he uses (if at all) and if there are GUI options to let the user adjust that (similar to what Howard Oakley points out in the case of AppTamer). If he does make use of it, but does not allow users to adjust, you could still consider if you want to change the kernel state via terminal command. Howard explains the pros and cons of doing that.

Yep. SATA will cap you at abut 600 MB/s, easily outperformed by TB3/4 and even TB2.

However, the simple way to get around that is to just ditch SATA for NVMe. These days, chances are it will be even less expensive too. There’s lots of great flash to be purchased with NVMe interface going from lower end (~800 MB/s, $84 for 2GB) to higher end (2.8 GB/s read bottlenecked by TB3/4, $130 for 2GB).

And if you do want to stick to the lower end, on a modern Mac you might as well opt for a USB 3.2 shell (like this one for $12) which will be almost $100 less expensive than a TB3/4 cases. TB3/4 really only comes into play when you buy fancier flash that actually delivers in the area between 1–2.8 GB/s or if your Mac has no support for 10 Gbps USB.

Thank you for pointing out that the SATA interface is the bottleneck. However, I didn’t understand where you were getting 600 MBps until I read this article on Wikipedia and am posting the salient portion in case it is also unclear to anyone else:

Third-generation SATA interfaces run with a native transfer rate of 6.0 Gbit/s; taking 8b/10b encoding into account, the maximum uncoded transfer rate is 4.8 Gbit/s (600 MB/s).

Yep. In general, when guesstimating throughput for data interfaces (USB, SATA, FireWire, Thunderbolt, Ethernet, DSL, cable modem, etc.), you can get a pretty good ballpark figure by assuming 10 bits per byte of user data. So you can just take the interface’s bit-rate and divide by 10.

It’s not precise, because things like data compression can reduce the number of bits per byte of user data and all protocols have overhead. But for a rough estimate, it is good enough for most purposes.

The only exception to this I can think of is for parallel buses (e.g. SCSI). They will transmit a complete word (usually 8, 16 or 32-bits) per clock, so you should multiply the clock speed by the word size to get the maximum throughput of the bus. But because those protocols also have overhead, dividing the total bits-per-second rate by 10 still presents a usually-good-enough estimate.

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Today, I tried to do exactly that with my wife’s iMac at work (Retina 5K, 27-inch, Late 2015). I have a Thunderbolt 4 enclosure (ACASIS 40Gbps M.2 NVMe SSD Enclosure with Built-in Cooling Fan) with a Samsung 980 Pro M.2 NVMe 2 TB SSD which works perfectly when attached directly to my M1 Mac Studio. I bought an Anker 5-in-1 Thunderbolt 4 Mini Dock to provide power for the enclosure (which also works between my Mac Studio and the ACASIS external drive). I attached the Thunderbolt 2 cable to the iMac and the Apple “Thunderbolt 3 to Thunderbolt 2 adapter” which I connected to the Thunderbolt input on the Anker Dock with the ACASIS connected to an output and … nothing. The hard drive did not mount. So I am wondering if the cable or adapter are faulty. I am going to try connecting to the Thunderbolt 1 port of my 2012 13" MacBook Pro which is at home. To be continued. :slight_smile:

I tried attaching my Thunderbolt drive to my MacBook Pro Thunderbolt 1 port using the same hardware and it didn’t appear on the desktop.

I now suspect that my Anker 5-in-1 Thunderbolt 4 Mini Dock is not compatible with the Thunderbolt 3 output of my Apple Thunderbolt 2 to 3 adapter.

If the enclosure you used is bus powered, it can not work with the Apple adapter as the adapter does not pass power,

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My Thunderbolt enclosure is bus powered, but I hooked it up to an Anker Thunderbolt dock which has a power supply and everything works fine with my Mac Studio. I think the Anker hub is incompatible with Apple’s Thunderbolt 2 to 3 adapter.

I have 2 RAID 4 volumes, one with 8 x 5400 RPM drives, one with 8 x 7200 RPM drives. Both in ThunderBay Thunderbolt 2 enclosures. The first volume tested (with BlackMagic Speed Test, while empty, ie, on the outermost and fastest edges of the disks) at about (lots of variation) 700/925 MB/s write/read speed. The second volume, when empty, tested at about 850/1150 MB/s write/read speed.

I also “certified” the 8 x 7200 disks using SoftRAID, which writes zeros to all disks simultaneously. In this case each drive is being addressed separately, with no overhead for file system, etc. For that I got a speed of 1.5 GB/s, which is faster than BlackMagic reports (850 MB/s).

So to copy from the slower (925 MB/s) at about 50% full to the faster (850 MB/s) at empty, with CCC, you might expect 700–800 MB/s if all went well. (The new volume will be using the outer tracks to write to, while the older volume will reading from various tracks across the disks, but mostly outer tracks because it isn’t full).

Judging from Activity Monitor, I get only 550–600 MB/s, or perhaps 75-80% of what I might expect, and probably less than 50% of the raw speed that the disks are capable of. I’m not complaining, just reporting.


Before I mothballed it, I was using a 2017 iMac (27 inch) as a fallback machine. When I first got it, I wanted to keep using the Thunderbolt-1, 1 Tbyte La Cie Rugged drive I had used on a previous, TB-1 only machine. I bought the Apple adapter and it worked fine.

Time Machine is certainly convenient for accessing older versions of files you’ve managed to mangle, but I was more concerned about an internal drive failure in the iMac, as I didn’t want ever to have to open it up, great iFixIt videos or not. So I sprung for Bombich Software’s Carbon Copy Cloner, which is (1) much faster and (2) absolutely brilliant for Intel Macs in as much as it allows to create and incrementally update a bootable clone. CCC costs $50, but if everything digital of any importance is on your Intel Mac, it’s just brilliant (as long as you remember to grab that portable drive when you’re evacuating your burning house).

Finally success in hooking up a Thunderbolt 4 hard drive to my wife’s 27" Retina iMac 2015 which has Thunderbolt 2. The OWC 14-port Thunderbolt Dock (currently on sale) worked using my Apple Thunderbolt 2 to 3 adapter and a Thunderbolt 2 cable attached to the computer. The hub provides power over the usb-c/Thunderbolt port to the 2TB external Thunderbolt drive.

The plan is to get some more mileage out of this computer which has a perfect 5K screen but only a 500 GB SSD internally. Initially, the external Thunderbolt M2 SSD will be used to store a large Photos library and for a Carbon Copy Cloner clone to a disk image (which will be unmounted between backups).
Doug Hogg

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Thanks for sharing this information. This is what I assumed you would need to do, but until now I haven’t read a first-hand account. So now I’m more confident in that assumption.