Help, cannot boot MBP from external SSD via USB...

Discussion in 'MacBook Pro' started by MK25toLife, Apr 9, 2018.

  1. MK25toLife macrumors regular

    Aug 7, 2007

    In the past, I have used an early 2015-MBP to boot up my external SSD via USB. Now, I am on a 2017 MBP running High Sierra and I can no longer boot up the SSD.

    I've tried two options; pressing "hold" during restart and selecting the SSD in the Startup Disk option. Both result in a circle with a slash through it. The ability to select the SSD is there in both cases but it won't boot.

    Has something changed since I'm using a new computer? Is there anything I can do get this working. Your help is very much appreciated!
  2. treekram macrumors 68000

    Nov 9, 2015
    Honolulu HI
    What OS version is on the SSD? It would have to be the 10.12.5 (Build version 16F2073, this is likely the special build for the 2017 MBP) or 10.12.6 or High Sierra. If the OS version is something lower, it won't run on the 2017 MBP.
  3. MK25toLife, Apr 9, 2018
    Last edited: Apr 9, 2018

    MK25toLife thread starter macrumors regular

    Aug 7, 2007
    Hmmm, I really don't remember, I think it might still be on El Capitan. Is there a way to upgrade my SSD if I can't even get it to boot?

    Edit: Looks like my SSD is on 10.11.6
  4. treekram macrumors 68000

    Nov 9, 2015
    Honolulu HI
    You can find out what version OS is on the SSD by opening the file on the SSD:
    [whatever the volume name is for the external disk]/System/Library/CoreServices/SystemVersion.plist

    You might want to check this because if you have a version on the OS that can boot on your 2017 MBP and it doesn't, you have a different issue. If you do have an OS that can't run on your external SSD, you can run the High Sierra installer on your 2017 MBP and target the external SSD. Before doing that, if there's anything important on the SSD you should back it up. You can download the High Sierra installer from the App Store. The simplest thing to do is to download it to your MBP disk, run the app and specify the external disk as the target.

    If the purpose of the SSD is to be a bootable clone (exact copy) of your 2017 MBP, you can use something like CCC (free for 30-day trial) to make a clone of your MBP disk onto the external SSD. According to the CCC website, it appears that as it is, assuming your external SSD is HFS+ (the file system before High Sierra), it will stay as HFS+. If you want to use APFS on the external disk you need to erase it as APFS beforehand.

    If you were already using CCC or another clone software, make sure it has support for APFS (CCC recommends you have CCC 5).
  5. Fishrrman macrumors P6


    Feb 20, 2009
    OP wrote:
    "Edit: Looks like my SSD is on 10.11.6"

    10.11.6 -IS NOT- going to boot a 2017 MacBook Pro.
    No way, no how.

    If you want to boot 2017 from the SSD, you have to upgrade the OS on it to a version that CAN boot the 2017.

    Tell us what you are using the SSD for.
    Is it intended to be "a backup"?
    Or do you keep "other stuff" on it (things that are NOT on the 2017 internal drive)?

    My suggest would be to use CarbonCopyCloner (or SuperDuper) to clone the MBPro drive to the SSD. It will then become a "bootable cloned backup" -- an EXACT COPY of the internal drive. VERY useful to have one around.
  6. MK25toLife thread starter macrumors regular

    Aug 7, 2007
    Thanks for everyone's help, the issue is resolved. I was using the SSD as an image of my old computer and needed to upgrade the OS to High Sierra to match the MBP I am using which is also on HS.

Share This Page