I have three WD external drives and I use WD Quick View to detach HDDs when I need to. What it does is, it finishes operations/flushes the drive, powers down the disk (stops the rotations), the LED goes off, and Windows chimes the drive removal sound. Essentially, the WD QV, stops power to the USB port. Is there any way one can do this to all the USB ports for flash/HD Drives?
You want to find the "Safely Remove Hardware" icon in the system tray, and when clicked it will provide a menu allowing you to eject individual devices:
In versions prior to Windows 10, the icon appeared as a USB plug with a green checkmark.
You can also right-click on a removable drive in Windows Explorer and select "Eject" from the menu:
I don't have enough reputation to comment, so I am just going to make an answer.
You can use USB Safely Remove, as it seems to have the functionality you are requesting like powering off USB flash drives and HDDs. Why you would need this for flash drives is beyond me, maybe you can enlighten me.