* NVS2500
NVS2500 does not designed to support USB HDD, so operation with external USB HDD is not guaranteed. NVS2500 may not be able to supply enough power to run some USB2.0 HDD.

* NVS2525
The eSATAp/USB2.0 host connector of NVS2525 is designed to be used with eSATAp HDD(Delock case included in NVS2525 Plus 1 package).
You can connect USB2.0 card reader or USB2.0 HDD, but that is extra function, slow copy speed because of USB2.0 speed limitation.
This eSATAp HDD case(Delock) is included in the NVS2525, but also available buying here.
http://www.nextodi.com/en/acc_store/acc_store.php
Note that, you need to connect Delock case via eSATAp cable included in NVS2525.
If you want to use USB3.0 HDD with NVS2525, read following.
NVS2525 USB2.0 can have problem with USB3.0 cable(not the US3.0 HDD itself).
Plug of USB3.0 cable has both USB2.0 portion and USB3.0 portion internally and that can cause problem when connected to USB2.0 connector.
If you want to use USB3.0 HDD, use USB2.0 micro-B cable.
This USB2.0 micro-B cable fits for USB3.0 micro-B connector adopted by most of USB3.0 HDD using 2.5’ HDD.
Google with [USB2.0 micro B cable] or at the Amazon for the seller.

* NVS2501/1501
Users can use USB2.0 HDD with NVS2501/1501.
But NVS2501/1501 USB2.0 can have problem with USB3.0 cable(not the US3.0 HDD itself).
Plug of USB3.0 cable has both USB2.0 portion and USB3.0 portion internally and that can cause problem when connected to USB2.0 connector.
If you want to use USB3.0 HDD, use USB2.0 micro-B cable.
This USB2.0 micro-B cable fits for USB3.0 micro-B connector adopted by most of USB3.0 HDD using 2.5’ HDD.
Google with [USB2.0 micro B cable] or at the Amazon for the seller

* NVS2825
Designed to be used with USB3.0 HDD. No problem at all.