Target Disk Mode extremely slow between two MacBook Pro

Here's how I worked around the problem.

1. On the new computer, I created a new volume "Backup".
2. I put the new computer into Target Disk Mode and attached it to the old computer using a Thunderbolt 3 cable (old computer is MBP 15" late 2016, new is 2018).
3. On the old computer, I logged into a "testbed" account on a "testbed" volume separate from Macintosh HD.
4. I used Carbon Copy Cloner to copy the old computer's Macintosh HD to the Backup volume on the new computer.

   This took about 105 minutes for about 850 GB of data

5. I rebooted the new computer and used Migration Assistant to copy the data from the Backup volume to Macintosh HD.
   1. Here is where some AFPS magic happened because I think much of my data was "copied" without actually having to move bits. I observe that the space occupied on the hard drive far less than doubled during the migration process.
   2. When the progress bar showed 75%, Migration Assistant reported 5 more minutes to go at 902 MB/s (yes, megabytes per second). After this things slowed down considerably and it actually took more like 37 more minutes. The whole migration took 52 minutes.
6. I am migrated. All said, this took well less than 3 hours for 850 GB of data.

Not a technical solution but a workaround; use Filesharing on your old computer and just connect thru Wifi or ethernet (much faster). Not pretty, but it does the trick in a reasonable amount of time.

Hello,

I often install new Macs using target disk mode for my Job and i already have this issue since using Thunderbolt 3.

I could see transfer speed near 20 MB/s (less than FireWire 400 !!).

Now i'm only using Ethernet Gb/s for transferring Data when it's possible (Adaptor needed for laptop) , but it's a waste of time ! We should have at least the Data Rate of the slowest disk .... nothing less than 120 MB/s ...

I could try different way, changed thunderbolt adaptors (apple, owc, Dock …), i try different apple cable as well as third party ones. I tried using migration assistant as well as finder copy or Carbon Copy Cloner.

I Tried both sides : old mac as target or new mac as target ...

My co-workers and I always had this issue....

I'm not sure but i think it was all TB1 computers (10 Gb/s ) , I'm trying to verify all experiences i had .

I guess it's a Compatibility issue between TB1 and TB3

It's a weird that we can't find more discussions about this !

And annoying that Apple seems to not know this issue.

Was having this problem too with a brand new 2018 Touch Bar MBP and a 2014 Retina MBP, Apple TB2-3 adapter, Apple TB2 Cable (tried an OWC one as well.) Really freaking annoying trying to migrate gigs of data. Ran the black magic disk speed test and it was abysmal. Did the same thing, and flip flopped which machine was in target disk mode and still no luck.

Seems like it's the finder process that's gunking it up somehow? Not really sure, but had super slow/irregular transfer speeds using finder and third-party app Pathfinder (I like to have verification for large transfers of crucial data.) It ran overnight and could not copy 100Gb.

Anyways, this is what's working for me now, while it may not be ideal, it's getting the job done:

Econ Technologies ChronoSync is now currently copying my photo library at 115 MB/sec, and it should be done in the next 10 minutes - with file verification turned on. If anyone is looking for an immediate solution to this stupid issue, check out ChronoSync - seriously it's moving 500Mb movie files like they're nothing.

Wow, it just finished in the time it took me to make this account and write this - 82.24GB in 11:52.

Hope this helps people stop banging their head against the wall (or their $5k freakin laptop that doesn't perform as expected.)

You are not alone. I'm working with level 2 support at Apple but have low hopes of a quick solution.

I have a 5 day old 2018 MacBook Pro i9 with a 2TB SSD and a 2013 MacBook Pro i7 with a 1TB SSD.

I'm using Apple's TB3 to TB2 (USB C to Thunderbolt 2) adapter and their Thunderbolt 2 to Thunderbolt 2 cable.

I put the old computer in target disk mode.

I can see the old computer's HD in Finder on the new computer.

When I open the old computer's drive in Finder, it takes a very long time for it to show the files, but they do appear (30+ seconds, possibly 60 seconds or more).

When I attempt to drag a folder from the old to the new, sometimes Finder is so slow it won't recognize the drag. I have to wait additional time before drag behavior works.

Once I drag, I get a "preparing to copy" dialog. The first time I tried it, I waited 2 hours before stopping. It seemed to be progressing in the file count, but I had no idea how far along it was and how much more it would take.

My transfer is going from a ~1000 MBps SSD to a ~3000 MBps SSD over thunderbolt. The whole 1TB drive should be able to copy in about 15 minutes.

I tried different ports. I did not try going the other direction (making the new MBP the target and copying to it).

I'm not encouraged by my Apple support conversation so far. No real answers. The suggestion is that a very long preparing to copy stage is normal, but that's not true. Either the old or new computer could index the whole drive in a few minutes, regardless of how complex it is.

Frustrating.

I've successfully used target disk mode in the past on older Apple hardware and it worked brilliantly.

In the meantime, I moved 35% of the data using an external disk. That took less than 90 minutes. And I saw none of the crazy slow preparing for copy behavior when I copied from the old machine to the external drive.

Hope we can find a solution. It should be blazingly fast.

Hello, it's not so bad if in your computers, you had Hard-drives, while you should hope to copy 850 GB in average 2 hours : ± 120 MB/s => 430 GB/h

But, seeing your "902MB/s" data rate i wonder if you had SSD? (given for 2,26GB/s in write mode)? So in this case, 3 hours is really slow ! Even if it's better than the direct solution...

Same exact issue here.

Thought I'd add some new info to the conversation.

TL;DR Version: Using 2013 MBP as the host and the 2018 MBP in target disk mode, Finder loaded the contents quickly, the "preparing to copy" phase was similar to an external drive experience and the transfer rate over thunderbolt averaged 440 MBps.

Details:

I couldn't wait on a workable / trustable target disk mode solution, so I used an external drive to transfer critical data and then backed up the 2013 MacBook Pro (Retina, i7, 1TB SSD) several different ways (just in case). The older MacBook is going to college with my daughter tomorrow so I had to wipe it, install a fresh OS, get basic apps setup, move her data to the laptop and setup backup on an external drive.

FYI - it took 40 minutes to move 377GB of my most critical data to the external drive and a tad under 40 minutes to copy it to the 2018 MBP. An hour and twenty minutes was way faster than the 2 hours of watching it "preparing to copy". Of course I attempted target disk mode many times and have now spent a few hours researching and a few hours talking with support.

I couldn't ignore the target disk mode issue. I picked up a Belkin USB C to USB A cable (certified for USB 3.1 data transfer) and tried 4 methods:

2018 MBP (Host) - 2013 MBP (Target Disk Mode)

• USB C - USB A Cable

  Doesn't work. The 2013 MPB apparently doesn't support target disk mode over USB. When in target disk mode, only the thunderbolt icon is shown on the screen. Finder on the host never sees the target drive.

• USB C Thunderbolt 3 adapter - Thunderbolt 2 cable
  • drive mounts. Finder takes between 20-120 secs to display the contents of the target disk. But it's actually not ready to work on the contents for a significant amount of time later (2-5 minute wait). At least, that's how long it was before the drag-to-copy UX became functional.
  • Preparing to copy phase is incredibly slow
  • I never waited long enough for Finder to begin the copy
  • I ran Blackmagic disk speed test on the target drive
    • In similar fashion, it took a long time (20secs plus) before the target disk shows any contents in the "select disk/folder" dialog of the Blackmagic app.
    • Transfer rates seemed stable at 390-405 MBps with no significant difference between read and write

      I was expecting/hoping for rates near 900-950 MBps as that's the speed of the slower SSD. Thunderbolt 2 should handle that easily.

2013 MBP (Host) - 2018 MBP (Target Disk Mode)

• Both cables work in this direction. The 2018 MBP shows both the thunderbolt and USB icons when in target disk mode. Interestingly, with both cables, the target drive mounted quickly and the Finder was able to present the contents of the mounted drive quickly. It didn't seem quite as fast as when I mount a typical USB 3 external HD, but it wasn't far off.
• USB C - USB A Cable

  Transfer rates measured using Blackmagic were between 100-110 MBps. Slower than I expected (USB 3 transfer rates are rated for 480 MBps). This wouldn't have been blazingly fast, but it would have saved time over the fiasco induced solution.

• USB C Thunderbolt 3 adapter - Thunderbolt 2 cable

  Transfer rates measured about 440 MBps, read and write.

Both Macbook Pros are running High Sierra 10.13.6 with SSDs formatted in APFS mode.

NOTE: When I first attempted to use target disk mode, the 2013 MBP also had a Boot Camp partition. I don't believe this had any impact, but thought I'd point it out in case others had questions.

It's not the i9. I'm have the same trouble with a 2018 MBP with 2.2 GHz i7. It does have the 2TB SSD.