external drives randomly un-mounting

I have a trashcan running Sierra 10.12.1

and my external drives randomly un-mount themselves. I power them down, unplug them, do restarts...sometimes it works, more often they just randomly come back on their own. Consistently get errors saying disks not ejected properly... I am not bumping them, or physically touching them at all.

any ideas on why this is happening?

the externals in question are:

Lacie -SAMSUNG HD103SI Media

FANTOM WD20EADS-00S2B0 Media

and another Fantom that just went offline on its own -- GreenDrive3

they are all usb connections

my system specs:

Model Name: Mac Pro

Model Identifier: MacPro6,1

Processor Name: 8-Core Intel Xeon E5

Processor Speed: 3 GHz

Number of Processors: 1

Total Number of Cores: 8

L2 Cache (per Core): 256 KB

L3 Cache: 25 MB

Memory: 64 GB

Boot ROM Version: MP61.0116.B20

SMC Version (system): 2.20f18

Illumination Version: 1.4a6

thanks for any insight!