[NPS-10] Considerations when choosing an external USB drive
The NPS-10's power system consists of an internal Li-poly battery (4000 mAH) and an external USB power source through a Type-C port. Internal battery powered system can supply 7-8W of power for 2 hours. The NPS-10 with internal SSD/HDD needs about 5W, and the memory card is usually less than 2-3W. Therefore, the NPS-10 battery power system can operate the main unit and memory card without problems.

When you want to make secondary copy of your backup, you can do it with NPS-10 through 'Sync' operation. Usually, external USB drive require additional 5W of operating power.
Because NPS-10 battery power system cannot provide 10W(internal 5W + external 5W), NPS-10 ask you adding external power through its Type-C USB connector.

When getting USB bus power through USB type-C port, NPS-10 limit the current draw from the USB host through the cable to 0.9A(when connected to computer) or 1.5A(when connected to USB battery or USB power cable). That limitation is according to USB3.0 spec(which limit USB power to 0.9A) and extended by USB type-C spec(which enable 1.5A or 3A). Theoretically, you can get 3A through Type-C port, but that require special handshake and well designed Type-C cable. If not, it can destroy your device as described below.

To avoid this kind of power disaster problem, we stick to baseline max 1.5A(5V) current draw from the USB type-C port as specified by USB Type-C standard.
As some body might noted above, 1.5A(5V) means 7.5W of power, and that is not enough doing 'Sync' which require 10W to run both internal & external drive. So the power system of NPS-10 is designed to draw additional 2.5W from the internal battery when required. So internal battery is constantly consumed during the 'SYNC' operation even though you connected external power through TYpe-C cable.

As with the same reason above, the maximum output current to external USB device through USB Host port of NPS-10 is limited to 1.5A(may be safe to less than 1.3A). If the peak current requirement of your USB device require more than 1.5A, it will not work properly with NPS-10.
So check the power requirement of your external USB is less than 1A when selecting your drive.
Big players (Seagate, WesternDigital, Samsung, etc) know on this USB power limitation, and no problem running with NPS-10.