Under no circumstances should you manually manage the filesystem. You will break things. (Ok, that's a bit dire, but it mostly holds true, we have extensive automounting in place, that will freak the fuck out if you try to side step it... if you're interested, look at /etc/auto.PlutoStorageDevices, if you want to understand the extent of it.)
Basically, any disk that is detected and/or added, is a device in the device tree. If you look in Advanced > Configuration > Devices, in web admin, you'll see your device.
You can alter its characteristics by clicking on it, and altering device data. Deleting the device, and reloading the router, will cause the disk to no longer be mounted, and the relevant directory symlinks deleted from /home
If you want to manually add a disk, you'll need to tell me a bit more, is this a local disk? In that case, #1790 would manually be added, if it isn't automatically detected?
Is it a network disk? If so, a File Server device would be added, with a child for each share (NFS or SMB). Again, this would automatically be detected, unless the file server had already been detected and you either configured or ignored it (it will not scan it again), in which case, you would probably just modify the children manually, adding, deleting them from the device tree.