For DSL 2750U router ( firmware version - IN_1.10 ) there is an option in router settings page under Advanced Setup->StorageService->UserAccounts to protect the usb storage with a user name and password.
Related links are
here and
here. Problem is, even if you configure, the usb storage is accessible
with out any restrictions to all users at \\routerIp\u_disk\usb1_1. About the username/password configured, I was never able to access drive with it.
I contacted regional dlink support person(India) over chat. He appeared less knowledgeable than a layman. Will try getting L2 technician over phone. Also emailed gplcode at dlink dot com for source code of DSL 2750U router. No replies yet. I feel it is best to read the source instead of contacting all these technicians who appears to have no clue. Even worse is the manual (download
here). If any thing is requires slightly complex configuration, manual will not mention it for sure.