You’ve got answers from some who know a bit more about it than I. On a tangent, here’s some info I’ve ciphered out. The tablo has an http server, the drive seems to be accessible on port 18080 and internals on port 8885 - or so this is my presumptive speculation. (note: you will need to know your own tablo’s IP or name on your network) Also note, none of this is anything you’d really do manually for any practical purpose. All this info is in JSON format, I use firefox which formats it into readable for me, otherwise you’ll need to search for a JSON format tool.
Your scheduled programs are http://tablo.lan:8885/guide/shows?state=scheduled Right, that doesn’t tell you much. Take one of those paths and get the show to be recorded http://tablo.lan:8885/guide/series/XXX The entire guide is at http://tablo.lan:8885/guide/series and of course you 'll have to break each of those down.
Note sure what you meant by “hidden channels”. Channel info it stored at http://tablo.lan:8885/guide/channels Each of those can goes further as http://tablo.lan:8885/guide/channels/XX
The drive listing is accessible via http://tablo.lan:18080/pvr/ now it doesn’t give you much as-is. Each of the directories represent an episode, http://tablo.lan8885/recordings/series/episodes/XXXX for specific info.
Again, none of this is practical, like this. I find it provides some insight. It can be useful if you care to write a shell script, or you can remember how to do string manipulation or work JSON to dig out some info, or stream shows through your own player or store them on another drive.
I think you may have some misgivings about transferring your files over the another drive… but it does vary by user.
http://tablo.lan:8885/account/subscription
http://tablo.lan:8885/server/harddrives
http://tablo.lan:8885/server/tuners
Be aware, using an old worn out drive might make give you problems 
with only one specific issue 
with your rig