External drive won't start/stop with computer

The external drive I use for Time Machine always worked perfectly under 10.7. After my 10.10 upgrade it wouldn't turn off when the computer was turned off, only when the computer was put to sleep. I have followed suggestions to reset the NVRAM and repair permissions as well as switching the computer/drive connection from USB to firewire, all to no avail. The cable switch produced the odd effect of reversing the bad behavior from only turning off for sleep to only turning off for shutdown. Suggestions?