TL;DR Formerly unusably-slow for backups a few years ago, the built-in Target Disk Mode in Macs has improved in latter-day Intel models (using a Thunderbolt 3 cable), now nearly 1/2 as fast as an external USB-3 drive dock.

My test: use Carbon Copy Cloner v5.1.22 to clone an internal SSD with 690GB of data. “Find & replace corrupted files” is OFF*.

Source: Macbook Pro 16”, Catalina 10.15.7

Targets: external SSD (usb 3) vs. 2020 13” MBP (TDM, thunderbolt 3)

Result: Direct-connect two MBP using a high-speed Thunderbolt-3 cable was 5.9 GB/minute, about half as fast as backing up via USB 3 to an SSD in an external drive dock (12 GB/minute).

Keep in mind that two “trips” are required if the goal is to move the data to a new Mac via an external SSD (first from source -> external SSD, then from external SSD -> target)… so when you add the time it also takes to restore the external SSD to the target Mac (which I did not measure), the net time difference between the two approaches is less.

For my purpose, simply cloning Mac#1 to Mac #2, the direct-connect using Target Disk Mode was simpler and adequately-fast. YMMV.

Target Connection Time (690GB)
External SSD, cheap Insignia Dual Dock USB 3 56:32
Internal SSD of a 2020 MBP 13" Intel (updated to Catalina 10.15.7) in Target Disk Mode, writing to a new APFS volume** Thunderbolt 3, direct connect, using 40G Pluggable cable plugged into Left*** ports 1:56:23
  • FWIW the time to use CCC’s “Find & replace corrupted files” feature just to confirm file integrity, with essentially no new data being written, was 3 hours using Target Disk Mode; e.g. 50% longer than the time to clone the disk to a new volume with the feature disabled.

** Because APFS makes it so easy, I now keep the “out of box” OSX in the original APFS volume when I get a new Mac (as a backup bootable volume, updated to the lastest version of Catalina), use Disk Utility to add an empty APFS volume for work, then clone my old drive to that new work volume.

*** I didn’t test it myself to quantify any different, but have seen claims that the right ports have lower data transfer speeds.