If I under the poster’s question correctly, I believe there is a simple way to switch from drive to drive just as he is asking. If you wait until Tablo is not recording anything (just to be safe) and quickly press the reset button, the Tablo will write all the required info TO the drive. Then, after the blue light stops flashing (I wait another minute of so to be safe) you unplug the drive from the Tablo. Then you do a factory reset on the Tablo. After it comes back up WITH NO DRIVE CONNECTED, then you connect the second drive and the blue light will begin to pulsate. After that stops (and it can take a LONG time if it’s a big drive with a lot of shows), all your shows, schedules, etc. will be there and ready to go.
It’s exactly the same procedure Tablo recommends to migrate your old recordings to a new Tablo. I have done it and I KNOW it works. Just remember, if you are using a drive with a lot of shows on it, it can take HOURS to complete the reload but it WILL finish.