I have an iMac 27 inch, and one summer the room temperature was up to 40C, and I came home that day, and the hard drive was really slow. I then rebooted the machine and it took something like 8 minutes to boot up since then.

So I suspect the hard drive is not working so well any more. Can I now use one of those Samsung T1 SSD USB drive, or a Seagate 2TB external USB drive, or a WD Thunderbolt My Book, and boot up the iMac using this drive from now on?

The thing is, how do I make such a bootable drive, when the original iMac isn't working so well? I do have a Mac Mini, a Macbook Air, and a Macbook Pro Retina, running Mavericks or Yosemite, so I do have some Macs to work with in order to set up this external hard drive.

(I supposed using the Samsung T1 would make the iMac really fast too? That's because I can connect the Samsung T1 to my Macbook Pro and copy files at a speed of 400MB/s)

P.S. and actually, since my Mac Mini (with USB 3.0) has an internal 5400rpm hard drive, can't I make such an external SSD boot up drive for the Mac Mini, and boot up my Mac Mini using this SSD from now on, and make the Mac Mini run a lot faster that way?

  • support.apple.com/en-us/HT201372 – n1000 Aug 28 '15 at 13:35
  • I'm going to link this to how to make an installer. This is just a) make an installer b) boot to the installer and install an OS on to the external drive of choice. The rest is user file migration and knowing how to change the boot order. – bmike Oct 30 '15 at 14:59
  • Use one of your other Macs to download the OS installer from the App Store for free.
  • Once it's fully downloaded connect the external hard disk you wish to install the OS (a format might be required) on and run the installer.
  • At some point the installer will ask you to choose a disk, by default the disk you are currently booted on is selected but you can change it by:
    • Clicking on the button under the disk icon
    • Select your external drive
  • Complete the installation.
  • In the meant time turn off the mac with the faulty hard disk
  • When the installation on the external disk is done:
    • Turn off the mac that you used to do the installation
    • Remove the external disk
    • Connect the external disk to the faulty mac.
  • Boot up the faulty mac while pressing down the alt button and a boot drive selection screen will be displayed.
  • Use your arrow keys to chose the external disk and press enter.

Once booted up you go to Settings>Startup Disk and choose the external disk so your mac always boots up from it by default i.e. you will not need to hold down the alt key the next time you restart your mac.


There's much easier and modern option - use createinstallmedia command in Terminal. Full guide for Yosemite.

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