Did you Enable File Sharing?
I'm probably stating the obvious here, and you guys have already covered it, but the problem could be that you haven't enabled file sharing and wifi sharing on your Playbook. You have to tap on the gear icon on the home screen, tap into "Storage and Sharing" and switch the "file sharing" and "wifi sharing" sliders to the "on" position. If file sharing is not enabled on the playbook, you get exactly the error you describe where you can see, but not access the file system.
Every time I've security wiped my device I've had to go back into settings and turn file sharing back on. There have been a couple of times that I forgot and got really frustrated with my computer until I realized what it was.
Hope that helps