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?