Remember this is Android not Microsoft Windows OS.
Q10/Q5Pro support standard Android unmount SD card
Settings /More / Storage
Identify you USB HDD (899 GB) use up/down to get it, and press unmount SD Card

Confirm unmount SD Card (OK).

Look parttion is unmounted (can mount back if need)

Repeat this step for all partitions, after unmounted all partition is safely to disconnect your USB HDD/Pendrive.
Quickly way like Nice Monkey said, power off box, disconnect USB HDD and power box back ~35 seconds.
LE: If anyone want to have unmount partition to home screen can try
MINIX METROhttp://www.mediafire.com/download/6d98s1061u3t0zd/MINIX_METRO.apk
