When I started running out of space, I got creative. Deleted a load of third party patches and that cleared up about 3GB so that was enough. Never downloaded Game & Wario after redeeming it in May with Mario Kart. Since then, all I've really been doing is downloading Virtual Console games and they're like 50MB each so plenty of space for them.
So far today, I have downloaded Game & Wario again, and Armillo which I have been wanting for a while but holding off. Got a load of eShop vouchers today, too, so don't have to worry too much about putting money on any time soon.
I'll check on that anomaly. I got the Seagate one I recommended to @yesteryeargames the other week after friends got it. Works wonders so far. Was concerned things would be iffy on the Wii U menu, but it works fast and seamlessly