Booting iMac (2011) off of external SSD I have been booting my iMac off of an external SSD.  When I initially set it up I did a clean install of high sierra and once that was booted up I used migration assistant to restore from a time machine backup.  It works well but one of the only issues that I'm noticing is that when I click on shutdown or restart the screen just goes black and the cursor stays on the screen but the system does not actually shutdown.  Any advice would be greatly appreciated.
The firsts things you can do :
Reset SMC : https://support.apple.com/en-us/HT201295
Reset NVRAM : https://support.apple.com/en-us/HT204063