I have attempted to improve the performance of a Mid 2011 27" iMac by booting from an external Thunderbolt SSD, specifically a Transcend 256 GB StoreJet 500. It may or may not be relevant that this does not have a separate power supply.

I have successfully installed High Sierra (10.13.2) on the drive, enabled TRIM and selected it as my startup disk in preferences, but when I first switch the computer on, it always boots from the internal hard disk. Only if I then login (which shows the external drive in Finder) and restart the computer does it boot from the external drive.

Also if I hold down ALT during the initial startup, only the internal disk is available.

I could live with this, but it's bugging me! Any idea what the issue could be, or any commands I can use to force the Mac to wait until the external drive is accessible before booting?

  • Have you partitioned this drive, or is it one large partition? – IconDaemon Jan 7 '18 at 15:54
  • One large partition – Matt Saunders Jan 7 '18 at 15:55
  • Please edit your question to indicate the version of macOS. Some research indicates that there may be problems with Sierra/High Sierra and booting from external drives, although I found nothing conclusive. If you had a drive with a power source, instead of bus-powered, you may have better results. Without any proof, I would guess that power is not available to the external drive until macOS boots. Not seeing it whilst holding down the Option key at boot points to this conclusion. – IconDaemon Jan 7 '18 at 16:07
  • Thanks @IconDaemon - added. I did my research and reviews suggested this worked well as an external boot disk, but you may be right RE power. Frustrating because otherwise it's working well. – Matt Saunders Jan 7 '18 at 17:13
  • If you computer will only boot internally, then the solution would be to install rEFInd on your internal disk. You could configure rEFInd to boot your external disk. The startup manager rEFInd could be installed into your existing internal EFI partition or a newly created 200 MB internal JHFS+ partition. – David Anderson Jan 7 '18 at 19:21

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