if you're going to plug the an external hd to the n810 via usb otg, you'll probably need a self powered usb hub depending how much power your external hd takes. as for canola browsing 30k songs, that will probably lag considering the n810 has limited cpu, 30k seems a lot, although i've never had that much so i'm only speculating

n810 gps is notorious for taking a long time to acquire the gps signal...but once you do switching between canola and gps shouldn't take long provided you're running both at the same time