I guess I didn't specify how I'm using ES File Explorer--it also gives me access to my PC over WiFi. I've also been connecting with a USB extension cable when I need access to the boot partition of the SD card, and oddly neither of the issues has recurred while using the extension cable.
I was also able to connect to my mom's Windows 7 32-bit machine with no problem when I first got the NC.
I've had some other USB oddities as well, so there's something going on in there.