Oh, and also, can you run Armory from the command line and tell me what happens? You'll need to go to the command line, go into the Armory directory, and manually start Armory. Here's an example. It assumes Armory is in /Applications, and that
/Applications/Armory.app/Contents/MacOS is your terminal's working directory. If your data is in the default location (i.e., you haven't moved anything around), ignore the datadir and satoshi-datadir portions.
./Armory --datadir=/Volumes/ExtDriveName/ArmoryDataLocation --satoshi-datadir=/Volumes/ExtDriveName/BitcoinCoreDataLocation
There are ways to automate this but focus on getting this to run manually for now.
ExtDriveName is actually whatever your drive is called, and
ArmoryDataLocation and
BitcoinCoreDataLocation are the directories where the respective data resides.