I was in the process of moving my iTunes library from one mac to a mini running server and headless. I was moving the iTunes library to a GDRIVE connected via thunderbolt. I followed all the steps posted online and went to start iTunes with the ""ALT/OPTION" key held down to point it to the new library nothing happened. I went through all the scenarios to try and fix it. Since I was on screen sharing I tried it on my MacBook, and the holding down the "ALT/OPTION" key worked fine. it wasn't screen sharing or my keyboard that was preventing iTunes from coming up with a switch library dialog. Then I changed the user from a limited user to administrator that didn't help. Then I tried using a different account and that made no difference on the mini running server. I think it may be a corrupt version of iTunes on my server, but want to try other things before I do something drastic like erase the whole mac mini and start over if its an OSX server bug. its been a long night with everything on the internet saying start iTunes while holding down "ALT/OPTION" and it doing nothing. Can someone running server and Sierra try pressing "ALT/OPTION" when starting up iTunes to see if the change library comes up?