Thats were it becomes a little tricky. It depends on what type of RNS you have. I have a verison "B" running firmware 1100, and with with this my RNS only support s 2gb SD card maximum. The newer RNS firmwares (2600 onwards) support 32gb SDHC cards. The reason I mention this, is that with my 2gb card, I can obviously only transfer a small amount of music to the RNS at a time. With the 32gb RNS, you can fill up the HDD in one go.
I try and mirror the file and directly structures on both card and RNS. I creat the folders on the SD card on the computer, create the sub folders as indicated, and then start filling up the card with albums.
The thing I have to watch is that if for instance, I already have a folder on the RNS (say, MUSIC/FEMALE ARTISTS/KYLIE MINOGUE, and I already have one of her albums in that folder on the HDD say, FEVER, then if I insert a SD card with another album from Kylie, I can't just copy the whole Kylie folder over as it will overwrite the folders and albums already contained within. What I have to do is highlight the new Kylie album on the SD card, click [COPY], the HDD will then open, I then browse to the MUSIC/FEMALE ARTISTS/KYLIE MINOGUE folder (where there will already be an album), then click [CREATE FOLDER HERE]. This will then copy the new Kylie album to the existing Kylie folder on the HDD (if you follow) ?
Now, if your device supports the larger cards, then you can create the whole folder structure at your lesuire on the PC (or Mac), then just insert the SD card and either forget about copying the music to the HDD and just use the SDHC card, or if you do want to move all the music over to the HDD, just select the root [MUSIC] folder you created on the SD card, and copy to the root of the HDD.
That's the beauty of using a root folder called music, in that the RNS will only let you select one folder to be copied at a time, so if you had your music spread out over the SD card in no particular order, it would take you forever to copy everything. Having the Root folder of [MUSIC] allows you to still only select one folder (the root MUSIC folder), but everything in that folders (all the sub-directories) get copied at the same time.
Just don't forget that if you can use a 32gb SDHC card in your RNS, the HDD only has just under 20gb capacity for music, so you can't copy everything across.
Sorry for the long winded post, but it was kinda difficult to explain in just a few words.
Hope this helps you ?