Target Disk Mode Slow - Why?

[EwanWhoseArmy]

Hi, today was my first time using Target Disk Mode and it took a lot longer than I expected - around 35 minutes to transfer a 75GB file.

In my opinion, it was pretty ideal conditions for a fast transfer between:
an M1 MacBook Pro (16-inch, 2021)
an M2 MacBook Pro (16-inch, 2023)
transferring a single video file of around 75GB
via a USB-C to USB-C cable, capable of up to 10Gb/s (AKA 1.25GB/s)

As both of the MacBooks are capable of up to 5GB/s, I assumed it would be the cable limiting the transfer speed to 1.25GB/s, and therefore the transfer would take around 60 seconds, maybe we can even say 90 seconds to be generous, but 35 minutes is nowhere near to what I was expecting - more like 35MB/s, or in the words of an Apple Keynote - 35x slower than it should've been.

Does anyone know any potential reason that would cause a transfer to be significantly slower than 1.25GB/s between these two MacBook Pros?

 

[joevt]

You are using this method?
https://support.apple.com/en-ca/guide/mac-help/mchlb37e8ca7/mac

The Share Disk option of Apple Silicon Macs is not the same as Target Disk Mode on older Macs. Target Disk Mode gives low level block access to the disk so you can do stuff like boot from it or repartition it. Share Disk acts like a network share.

Here's an article about it but it uses a Thunderbolt cable:
https://eclecticlight.co/2020/12/04/how-fast-is-an-m1-target-disk/

Here's another:
https://eshop.macsales.com/blog/75872-how-to-use-share-disk-mac-disk-sharing-on-an-m1-mac/

A USB-C cable can do Thunderbolt at 20 Gbps as long as there's no type-A adapter involved. That should allow 1500 MB/s. In System Information.app, Does the other Mac appear as a Thunderbolt device or a USB device or something else? Check all the tabs. What link speed does it show in the Thunderbolt tab or the USB tab?

 

[kitKAC]

You should be using a Thunderbolt 3 cable for the transfer.

https://www.apple.com/uk/shop/produ...fdf9f66cf05b3adfe84de72930bb379c1b80c60d21d78

 

[EwanWhoseArmy]

You are using this method?
https://support.apple.com/en-ca/guide/mac-help/mchlb37e8ca7/mac

The Share Disk option of Apple Silicon Macs is not the same as Target Disk Mode on older Macs. Target Disk Mode gives low level block access to the disk so you can do stuff like boot from it or repartition it. Share Disk acts like a network share.

A USB-C cable can do Thunderbolt at 20 Gbps as long as there's no type-A adapter involved. That should allow 1500 MB/s. In System Information.app, Does the other Mac appear as a Thunderbolt device or a USB device or something else? Check all the tabs. What link speed does it show in the Thunderbolt tab or the USB tab?

That was indeed the method I used - Utilities>Share Disk - Sorry, I didn't realise they'd now changed the name of the method, but it's definitely what I did.

The cable I was using was USB-C with no type-A adaptor involved. Based on the website it can do up to 10Gbp/s (as opposed to 20Gb/s), but I don't think it's unfair for me to have expected at least roughly 1GB/s from that in a real-world scenario. To use your point of reference, I believe 20Gb/s should allow up to 2500MB/s (was 1500 a typo, or did I miss something?), and 10Gb/s should allow up to 1250MB/s, right?

Unfortunately I don't have the M1 Macbook readily available so I can't check it with the System Information app anytime soon, but I'll bear it in mind for next time I do this!

You should be using a Thunderbolt 3 cable for the transfer.

My cable can do up to 10Gb/s, and isn't the main relevant difference (in this case) with Thunderbolt 3 is that it can do up to 40Gb/s instead?

So ultimately I wasn't expecting the transfer to finish in 15 seconds at 40Gb/s because my cable isn't a Thunderbolt 3 cable, I was just expecting something closer to 60 seconds at 10Gb/s - not 35 minutes.

Surely Apple doesn't require it to be strictly with Thunderbolt branding to get a somewhat decent transfer speed? I hope that's not the issue anyway.

 

[joevt]

The cable I was using was USB-C with no type-A adaptor involved. Based on the website it can do up to 10Gbp/s (as opposed to 20Gb/s), but I don't think it's unfair for me to have expected at least roughly 1GB/s from that in a real-world scenario. To use your point of reference, I believe 20Gb/s should allow up to 2500MB/s (was 1500 a typo, or did I miss something?), and 10Gb/s should allow up to 1250MB/s, right?

Take the bandwidth and multiply by ≈ 80% to get a reasonable estimate of achievable read/write speed.

A USB-C cable has 4 wires. USB 3.1 gen 2 only uses 2 of them - one for receive and one for transmit. 10 Gbps USB uses 128b/132b encoding which is 9.7 Gbps of data = 1212 MB/s but you usually get around 1060 MB/s max.

Thunderbolt uses all 4 wires, two for receive and two for transmit. A USB-C cable can do Thunderbolt at 20 Gbps (10 Gbps per line). The bits are transmitted using 64b/66b encoding at 10.3125 Gbps per line (20.625 per line in a Thunderbolt 3 cable). 20 Gbps is 2500 MB/s but 1500 MB/s is closer to the actual amount of data you can transmit to/from a disk connected as Thunderbolt with a USB-C cable (or a Thunderbolt 2 cable).

isn't the main relevant difference (in this case) with Thunderbolt 3 is that it can do up to 40Gb/s instead?

Thunderbolt 3 usually can't transmit more than 22 Gbps (2750 MB/s). Sometimes up to 25 Gbps (3125 MB/s).

Surely Apple doesn't require it to be strictly with Thunderbolt branding to get a somewhat decent transfer speed? I hope that's not the issue anyway.

We won't know for sure without more info.

 

[InBruges]

Scenario here:

MacBook Pro (M1 Max, Running Sonoma) in Share Disk Mode connected with an Apple Thunderbolt 3 cable to a Mac Studio (M2 Ultra), connected as administrator via network as on Apple's guide

Result: 3 Hours to copy 79GB, so slow!

My USB 3.1 NVMe in a cheap USB C enclosure is way faster.

What's happening here?

 

[solouki]

Two comments:

(1) USB-C cables are notoriously sensitive to their advertising ... it has been found that some companies's cables, even if listed as 10Gbps, don't live up to those speeds. (I know nothing about the fairphone cable the OP is using, however.). On the other hand, I have a Thunderbolt-4 to Thunderbolt-4 cable and get pretty fast file transfer speeds across it.

(2) How was the OP transferring the 75GB file? Sure, the individual network data blocks themselves are checksummed on both ends, but some methods of transfer perform a checksum on the original file before beginning the network transfer, and then after the transfer another checksum is performed on the full-sized file --- these checksums on the full 75GB files may take some time, but I think that 35 minutes is a little extreme even for this. (For instance "rsync" is also notorious for having a low bandwidth and thus thus taking a long time even over a fast cable.)

 

[chrfr]

Hi, today was my first time using Target Disk Mode and it took a lot longer than I expected - around 35 minutes to transfer a 75GB file.

In my opinion, it was pretty ideal conditions for a fast transfer between:
an M1 MacBook Pro (16-inch, 2021)
an M2 MacBook Pro (16-inch, 2023)
transferring a single video file of around 75GB
via a USB-C to USB-C cable, capable of up to 10Gb/s (AKA 1.25GB/s)

As both of the MacBooks are capable of up to 5GB/s, I assumed it would be the cable limiting the transfer speed to 1.25GB/s, and therefore the transfer would take around 60 seconds, maybe we can even say 90 seconds to be generous, but 35 minutes is nowhere near to what I was expecting - more like 35MB/s, or in the words of an Apple Keynote - 35x slower than it should've been.

Does anyone know any potential reason that would cause a transfer to be significantly slower than 1.25GB/s between these two MacBook Pros?

Target Disk Mode on Apple Silicon Macs is just using SMB file sharing. It’s not like on older Macs where the disk would just work as an external disk, and it’s not going to be anywhere close to the max speed that the cable allows, but as mentioned above, Thunderbolt cables will be faster.