Thanks Joe, I'm gong to post Etre check results here. Bad or loose connection or cable between the power supply and the drive.Bad or loose power connection or cable between the drive power supply and the wall.The most common causes in my experience are a bad data cable, bad data connector/connection on the Mac or drive, bad power which could be the result of any number of causes including: I definitely would want to run down the cause of the disconnections however. Personally I wouldn't worry too much about damage, but it would not hurt to run Disk Utility > First Aid just to be sure. If not then one of the industrial strength disk utilities (Drive Genius, TechTool Pro, DiskWarrior) might be needed. Disk Utility is capable of detecting and in most cases correcting such errors. Fortunately the window for such an error occurring is very narrow and therefore the likelihood of damage is small quite small.

The danger with a drive not being properly disconnected is when it occurs concurrently with the computer writing to the drive which can create an error in the data.

The only diagnostic used in that case was a combination of years of experience (I had seen connecting cables go bad in the past), cost to repair (the cable was the least expensive item to replace), and intuition ( ie. I ran into a similar problem a few weeks ago and it turned out to be a connecting data cable that had gone bad.