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I use a MacBook Air, but would like to put an iMac at my office for a performance boost. I don't want to switch between operating systems, so I'd like to target the Air's SSD (via Thunderbolt 2) when working from the iMac. Here are the devices:

  • 2015 13" MacBook Air i7, 2.2GHZ, 8GB RAM, 512 SSD (Thunderbolt 2)
  • Retina iMac i7, 4GHZ Quad, 32GB RAM, (Thunderbolt 2)

My dream is that I'd be using the iMac's RAM & processor, and that Thunderbolt 2 would be fast enough that I wouldn't lose any SSD performance.

For me latency matters more than throughput, but I'm interested in both. Processing thousands of small files at once, not editing video.

Would this work? (theory, experience, benchmarks welcome)

UPDATE: I'm adding a bounty for anyone that can offer actual benchmarks of targeting a MacBook Air (or pro) SSD from a retina iMac (via thunderbolt 2).

To be totally clear - I'm only interested in "Target Disk Mode" performance. I want to run the Air's OS. I already know about file syncing, external drives, etc. Thanks everyone for the ideas though.

  • 1
    According to the Wikipedia article on Targeted Disk Mode, you use it when you do not want to run that computers operating system. The computer booted in Targeted Disk Mode acts as a Mass Storage Device. So your clarification is a little bit confusing. Are you trying to say that you want to use the iMac as a "Dumb Terminal" to control the Air? – AMR Jul 28 '15 at 16:29
  • @AMR Maybe it's not possible, but I had read that you could boot an external drive. I can see how it might be dangerous booting on a totally different setup. How To Geek says "With Target Disk Mode, you can treat a Mac’s internal drive as an external drive and boot from it, just as you’d boot from a typical external drive. This actually lets you boot the OS X system from one Mac on another Mac." – bendytree Jul 28 '15 at 16:38
  • That just sounds like a really bad idea. 80% of the article is about how you use it to transfer files. They mention the part at the end, but put the warning up that the systems should likely be identical. I can imagine that system settings for the Air and the iMac are different, even if you are running the same Flavor of OS X. Plus you will have to undo Fire Vault encryption, which means that your data is exposed. – AMR Jul 28 '15 at 17:23
  • So the question becomes Why do you want to do this? What benefit are you looking to achieve? Is there software on your Air that you want to run on your iMac that you don't have license for? Are you looking for the speed advantage of SSD? And if so why wouldn't a Thunderbolt or USB 3.0 SSD be a preferable alternative? You could even set that up as a bootable disk and boot your iMac from that drive. I doubt seriously that you will get better performance from the peer-to-peer connection than you will from a dedicated SSD. – AMR Jul 28 '15 at 17:27
  • And if it is because you just want to be able to control the OS of the Air from your iMac, then Screen Sharing lets you take over complete control of the other computer in a separate window. You can use the input and control devices of your iMac to control the Air. It is pretty seamless. – AMR Jul 28 '15 at 17:31
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So I tried it out and wanted to share my findings. First off, the equipment I was using:

  • Early 2015 13" i7 MacBook Air, 8GB Ram, 500GB SSD
  • Late 2015 27" i7 iMac 5K Retina, 16GB Ram, 1TB SSD

I restarted the Air in target disk mode & plugged it into the iMac via Thunderbolt 2. It wouldn't boot. After a few minute, it just showed a ban icon (circle with a line through it).

I ran some 5GB Blackmagic Speed Tests to gather transfer rates (in MB/S):

  • iMac against its SSD - 1817 Read, 1510 Write
  • Air against its SSD - 300 Read, 241 Write
  • iMac & Air against USB3 Flash Drive - 245 Read, 185 Write
  • iMac & Air against USB3 External Drive - 160 Read, 170 Write
  • iMac using Air via Thunderbolt 2 shared folder - 400 Read, 490 Write
  • iMac using Air in target disk mode - 79 Read, 107 Write

So obviously there's a big performance loss for target disk mode.

If you're wondering why I'd want to target the air, it's because I mainly work off my Air and don't want the hassle of an external drive, thumb drive, file syncing, etc. I love the idea of getting to my office and "docking into" a faster processor, more RAM, and a big screen.

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In theory you should not loose too much speed. Most modern SSDs utilize SATA 6 Gbit/s SSD controllers which support 500 MB/s read/write speeds. Thunderbolt 2 works at a theoretical 20 Gbit/s. So Thunderbolt2 should not be the bottleneck.

However.. Macworld did a performance analysis in 2011. It was a test on Thunderbolt 1, but it should still might give you some insights. If you go to Macworld - More Thunderbolt speed results and scroll down to the section "Benchmarks: Target Disk Mode via Thunderbolt" you see that performance did take a huge hit. Even if you double the speed for Thunderbolt 2, you'd still loose over 50%.

Then there is the more recent (2014) The Instructable article High-Speed Data Transfers between Macs with Thunderbolt, which says:

Thunderbolt is capable of faster speeds though this upper limit is a combination of the maximum read/write speeds of the Mac’s SSD and that Parallels VMs are split into many smaller files. ↩

Sorry no definitive answer.. And either way I'd recommend against this setup. I use Dropbox and/or Google Drive to sync files between my machines. This works well for me.

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So basically I hear three things in your Question.

  1. You want the speed and performance of the Air's SSD drive.
  2. You want to want to leverage the processing performance of the iMac.
  3. You want to access those files away from the office on your Air.

You should be able to setup a connection using a Thunderbolt Bridge under your Network settings. What follows is a reprint of Apple Help in Yosemite:

Use IP over Thunderbolt

Connect two Thunderbolt-equipped Mac computers using a Thunderbolt cable, then use Internet Protocol (IP) to communicate between the computers.

  1. Connect a Thunderbolt cable to the Thunderbolt ports on the computers you want to connect.

  2. Choose Apple menu > System Preferences, then click Network.

  3. Select Thunderbolt Bridge in the network connection services list.

  4. Click Advanced, click DNS, then enter the DNS and search domain addresses if your network requires them. Check with the network administrator or your ISP if you’re not sure what the addresses are.

  5. Click OK, then click Apply.

When you connect computers using IP over Thunderbolt, by default, the computers receive IP addresses using DHCP. To enter an IP address manually or select another way to receive an IP address, use the Configure IPv4 pop-up menu.

Next you will have to go into settings and set up file sharing.

Share your files with other Mac users

You can share files and folders with others on your network. You can share your entire Mac with everyone, or allow specific users access to only certain folders.

  1. Open Sharing preferences if it isn’t already open (choose Apple menu > System Preferences, then click Sharing).

  2. Select the File Sharing checkbox.

  3. To select a specific folder to share, click Add at the bottom of the Shared Folders list, locate the folder, select it, then click Add.

    The Public folder of each user with an account on your Mac is shared automatically. To prevent a folder from being shared, select it in the Shared Folders list and click Delete .

  4. By default, any user set up on your Mac in Users & Groups preferences can connect to your Mac over the network. A user with an administrator account can access your entire Mac.

    To give only specific users access to a folder, select the folder in the Shared Folders list, then click Add at the bottom of the Users list. Then do one of the following:

    • Select a user from Users & Groups, which includes all the users of your Mac.

    • Select a user from Network Users or Network Groups, which includes everyone on your network.

    • Select a person from your contacts. Create a password for the person, then click Create Account.

  5. To specify the amount of access for a user, select the user in the Users list, click the triangles next to the user name, then choose one of the following:

    • Read & Write: The user can see and copy files to and from the folder.

    • Read Only: The user can view the contents of the folder but can’t copy files to it.

    • Write Only (Drop Box): The user can copy files to the folder but can’t view its contents.

    • No Access: The user can’t see or copy files from the folder.

  6. OS X allows guests to access shared folders on your Mac. To turn off guest access, deselect “Allow guests to connect to shared folders” in the Guest Account pane of User & Groups preferences.

At this point you should be able to go into Finder on the iMac and mount the drive on the Air. Once mounted, the iMac can control and access the drive as it would any other networked drive.

I bring up 3. because otherwise the obvious alternative suggestion is just get an external SSD for the iMac. You will run into the same latency issues with an external SSD than you will the the Peer-to-Peer set-up you are suggesting, maybe less as the external SSD will not be running into all of the services that will be running on the Air.

However, it can be a drag carrying around an additional drive when the whole point of an Air is its thinness and portability. The alternative I have used for my MacBook Pro, more for data storage, but it might be an interesting option for you, is to use a SanDisk Cruizer Fit. I have found that they perform as seamlessly as my SSD with no noticeable performance loss, and my experience is with USB2.0. They now have USB3.0 versions up to 128GB. The Cruizer fit is perfect in the sense that it only extends a few centimeters past the edge of the case, and as a result can be left plugged into the Air without the risk of it snapping. This will likely give you similar performance to the other options and just swapping the USD drive allows you the ability to use your data between both systems.

  • Great ideas - thanks. I updated my question to be a bit more clear. Right now I'm really just trying to understand "target disk mode" performance over Thunderbolt 2. – bendytree Jul 28 '15 at 16:22

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