External HDDs connected to USB on router

[#1]

I have a Plusnet hub and an external HDD case, in which there are 5 HDDs (three physical, partitioned to 5). I have a home network and want to connect these HDDs to the network, rather than just to one PC. I want to do this by using the USB port.

I have successfully connected the two devices and found the HDD on the network. Rather, I have found 2 of the 5. The other HDDs are not showing up. 

Is there a limit to how many external devices are supported in the manner? If not, how do I see/connect the remaining drives?

[#2]

What file systems are on your external partitions?

In any case, you are not going to get a great throughput - the Plusnet router isn't exactly a powerful processor with loads of memory.

[#3]

@Tootiki 

Variants of your question pop up here regularly (about USB storage at least).

Personally I would buy a "proper" NAS case (A Synology would be my recommendation).

I've followed the path of shared storage for some years. I started with "LAN Disk" cases (from Maplin) and Buffalo & WD NAS units before ending up with the Synology.

Why do you need multiple partitions and how big are the drives ?

Is the cost of the solution a real problem ?