When I navigate on my desktop computer to my nas it shows two folders which I assume are the two drives. One is media with music etc the other called backup which I presume is the second HD.
When I navigate on my desktop computer to my nas it shows two folders which I assume are the two drives.
Your assumption is incorrect. Those are two network shares (shared folders) that the NAS creates by default. You can create additional shares by using the NAS admin interface ( https://nas-ip-address/admin - using the real IP address or hostname of your NAS). They aren't tied to a drive, both are stored on your RAID-1 (mirrored) array. The RAID mirroring that the NAS does is not visible from windows. You only see the one file system. Every disk write is simply done to both disks in parallel.
If you had more than two disks, the RAID becomes more complex (and isn't done with mirroring), but you still just see one volume (a virtual disk) with shared folders.
So what happened is that you just loaded all of your files into one folder.
You can check that the volume is redundant by looking at the NAS volume status - either using the web UI as I mentioned above, or using a utility called RAIDar ( https://kb.netgear.com/20684/ReadyNAS-Downloads#raidar )
You should take some time looking over the hardware and software manuals for your ReadyNAS. If you tell us the firmware it is running I can direct you to the appropriate downloads. The product labels are misleading on some NAS, the firmware version is an easy way to overcome that. Knowing the firmware will also tell us if you need a firmware update.
That is a v1 product (with a sparc-based processor). The current firmware is 4.1.16 - you should be able to update it using the web ui's check-for-update control (click on system in the left pane menu, then update). The newer firmware has a couple of security fixes that you are missing.
