Thunderbolt: not quite all daisies

Thunderbolt is blazingly fast, and simple to use: connect it up, and away you go. You can daisy-chain up to six peripheral Thunderbolt devices from each port on your Mac, and as few users have that many, there aren’t any complications, are there?

Well, much of that is true, but it does get a bit more complex than that.

Details reported on external Thunderbolt ports.


Let’s take a ‘simple’ problem which I struggled with today. I have a dead iMac, which still works fine in Target disk mode, and has two Thunderbolt ports. I have a MacBook Air with a single Thunderbolt port, to which I can connect a dock, which has two Thunderbolt ports, one of which is needed to connect it to my MacBook Air. I want to copy the contents of the hard drive in my dead iMac to a portable 2 TB hard drive, which has a single Thunderbolt port which also provides its power. I also have a fairly full Promise Thunderbolt RAID enclosure, with two Thunderbolt ports.

Despite having five devices with a total of eight Thunderbolt ports between them, and three Thunderbolt cables, it is not possible to connect them up so that the MacBook Air can see both the iMac’s hard drive, and the portable 2 TB drive. The obstacle to this is that some Thunderbolt devices have to go at the end of the daisy-chain:

  • Macs running in Thunderbolt Target (disk) mode,
  • Macs running in Thunderbolt Target display mode (TDM),
  • Thunderbolt devices with a single Thunderbolt port (by definition),
  • the sixth and last Thunderbolt device in each daisy-chain.

It is the first of those, coupled with the single port on my MacBook Air, which makes my problem insoluble. The only way to connect both the iMac and the external hard disk to the same Mac would be for that Mac to have two Thunderbolt ports, for the dock to have three, or for both the Thunderbolt ports on the iMac to be usable, so that it would not have to be at the end of the chain.

One natty solution might be a Thunderbolt hub (rather than a dock) with three or more Thunderbolt ports, but as far as I can tell, no such beast exists (yet). Another might be a (new) Mac Pro, with its astonishing total of six Thunderbolt ports.

There are other peripherals which benefit greatly from being alone on their daisy-chain, including:

  • high resolution Thunderbolt displays, because of the bandwidth they consume on the Thunderbolt bus,
  • second and subsequent Thunderbolt displays of any resolution, for the same reason in aggregation.

Some iMacs can be used in Thunderbolt display mode (TDM), which is different again from Target disk mode. Apple provides further information about this, where it does mention that you cannot access other devices connected to a Mac being used as a display.

One final practical note: as Thunderbolt is an active bus system, to use a peripheral which is further down a daisy-chain, all those between your Mac and that peripheral will need to be powered up to enable signals to be passed through.