If you’ve more than one Mac, you’ll often want to transfer chunky files or large folders between them. This article looks at which methods are fastest when at least one of those is an M1 model.

By far the simplest, and ideal for smaller files, is AirDrop. So long as both Macs have Wi-Fi and Bluetooth running and are fairly close to one another, it’s quick and convenient. It’s also about as slow as you’ll get, with transfer rates of a steady 40 MB/s between most recent Mac models, including my M1s. Once you need to transfer anything larger than 1 GB, you’ll probably prefer something faster.

Next up could be wired Ethernet; if you can hook the Macs up back-to-back you should benefit from 10 Gb/s, but all too often using an existing wired network only supports 1 Gb/s and you’re likely to see transfer speeds little better than those of AirDrop.

If you want to keep both Macs running normally, the fastest option is going to be Thunderbolt Bridge networking. Connect the two Macs with a Thunderbolt 3 or 4 cable, then open the Network pane on each. Normally, this will result in each getting a self-assigned IP address on the same subnet. However, if you’re going to use this more than occasionally, you’ll probably want to give them their own IP address and subnet mask. You may also want to move that connection to the top of the list, so that it should be used preferentially.

When used between two M1 Macs, you may find this temperamental, and transfer speeds capricious. If you click the Advanced… button at the foot of the pane, you can check which Thunderbolt port has been recognised. If you’re tempted to change its hardware configuration to use Jumbo frames with 9000 bytes of payload, I suggest that you don’t, at least not to begin with.

With automatic frame sizing, you can see very fast transfer speeds. When copying files of 1-10 GB size to an M1 Mac sharing files, I measured rates of between 500-700 MB/s, and copying files from the server was even faster at 700-1000 MB/s. Measuring speeds over the wide range of file sizes from 2 MB to 2 GB using Stibium gave a read speed of 1.0 GB/s, but write of only 380 MB/s.

Trying Jumbo frames generated a real mess. Straight Finder file copies moved very quickly until the final few MB, then took many seconds to complete. Stibium was able to measure similar transfer rates to those obtained with automatic frame sizing, with a read rate of 1.0 GB/s again, but a lower write rate of 360 MB/s. On another occasion, Finder copying dropped as low as 66 MB/s. I therefore don’t recommend that you alter frame sizing, as there seems no benefit, and Thunderbolt transfers could even slow as a result.

The final method of transferring files to and from an M1 Mac is its new Target Disk mode. When I last tried it well over a year ago, it still had some serious bugs, and could put the host Mac into an endless restart loop. As a result, while it was effective, it was also delicate. Apple has addressed these problems, and it’s far easier to use.

Shut the Target Mac down and connect it to the Host using a short Thunderbolt 4 cable. Then start the Target up in Recovery, holding the Power button pressed until it loads Options. Select Options and click Continue under it. Once the main Recovery mode window has loaded (offering its four options), open the Utilities menu and select the Share Disk command. Work through the sequence to select the disk until it offers you a button to turn sharing on; click that.

The Host Mac should now be able to see and connect to the Target using SMB. Transfer rates are the most impressive of all: a read rate of 1.2 GB/s and a write rate of 1.9 GB/s using Stibium. These also appear far more reliable and consistent than Thunderbolt Bridge networking.

The winner is definitely Target Disk mode over Thunderbolt, with the runner-up Thunderbolt Bridge networking when it doesn’t play up.

This leaves me with one big question: given that Thunderbolt 4 should be capable of 40 Gb/s, and even allowing for DisplayPort bandwidth (which is hardly likely in the case of Target Mode), we surely should see overall transfer rates of around 32 Gb/s. Making no allowance for encoding overhead, which is heavy for ethernet packets, that means we should start with transfer rates of around 4 GB/s gross. Thunderbolt Bridge networking delivers at best a quarter of that, and Target Mode slightly less than half. Once again most of the bandwidth of our Thunderbolt connections seems to vanish into overhead.

For external SSDs, Thunderbolt 3 delivers a maximum transfer rate of 2.8 GB/s, which is 70% efficient. Thunderbolt Bridge networking is far worse, with an efficiency below 25% on a good day.