Hi, I have a few Macs (two running El Capitan, and one running Sierra). The Sierra machine was purchased a couple of months ago, and it has a bizarre problem through a couple of OS updates: when any external disk is connected to the computer via USB (either NTFS or GUID) the system randomly drops the drive, or has a disk initialization error, or just becomes unresponsive on disk I/O with associated pinwheel. When this happens, shutdown/restart takes forever, hanging on the spinning icon on the black background, for anywhere up to several minutes. This happens any time ANY external disk is connected. I use these external disks also on the El Capitan machines without any problems. Time Machine also goes nuts on the Sierra computer, working for maybe one or two backups, then it complains it can no longer access the disk. I've tried various resets of NVRAM and PRAM, to no avail. Just what on earth is going on with Sierra, and does anyone know of any fixes? It only affects the Sierra computer. No problem in over 2 years on the El Capitan systems (both upgraded from Yosemite). Having a look around, it seems this problem existed since the Betas, and no resolution was found. I even tried downgrading the USB version from 3.0 to 2.0 via a USB 2.0 hub. No change in behavior (but slower drive throughput). Arrggghhh!! Thanks.