russ_fae_fyvie, sorry I've only just seen this post. Just a quickie reply and will answer better tomorrow, if needed.
First off, I think what you are using is a com link and not a usb, with the SP3
Second, when you 'download' your routes to the device make sure you have selected all of them, I'm not saying that you have'nt, only that this is a common mistake when downloading and only getting one route.
Third. When you download new routes to the memory card you overwrite the previous routes so there is no need to delete them.
Hope this helps but if you do need further explantion, then pop up another post.
I'll try better next time