Brand new mini having problems with external SSD enclosure

All was working fine out of the box, with an external drive enclosure and fast chip inside. Then out of the blue the mini won't do a restart properly... after sign in, the progress bar goes about 1/20th of the way, stalls, then loops. If I try with the enclosure unplugged, progress still goes 1/20 of the way and stops for a few seconds and then boots up ok. Arg, it all was working untill I loaded the SSD with things like a 266 GB photo library. Any thoughts? thx!!!