I have tried everywhere for this answer but all suggestion fail. I use a network attached External hard drive for open storage in an internet cafe. How can I real time protect this drive against infections. None of the major software programs seem protect network drives.
A network drive is seen as just a hard drive to the rest of the network. The only protection is to ensure that all computers on the network have AV software installed. Not really feasible in an environment where anyone can come along and attach to your WiFi.
There is no software that can run "on" a network drive as there is no standard operating system for these drives (some run Linux, some run Embedded Windows, some other custom software, etc), and they don't have the processing capacity to be able to cope with it anyway.
You would be better off having a shared area on a computer that is running anti-virus software. That can both protect itself and perform regular scans of the shared area for better protection. Yes, it costs more than a simple NAS, but it is more secure.
If you mean that you have a standard hard drive plugged into a router of sorts then the above post is correct you cannot. However if you have a real NAS attached to the network then the above statement is wrong in the assumption that there is no products but they may only work on specific NAS devices. Norton and Trend Micro offer antivirus programs that run on the NAS or will at least scan the files being requested by the NAS. Buffalo Technologies also have NAS's that have linux with Trend Micro security built into it. But in your case it would be simplest to just have an old computer running some light OS and just install antivirus on it and tell it to scan the NAS.