I had previously plugged in an external HDD and it seemed my recordings on the internal drive were not accessible. Looking at the post: All recordings on Tablo internal storage deleted after I plugged in an external storage device It seemed that this was how things worked.Well I had delete to make space turned on and lost some older recordings, so I decided to try the HDD again. This time the old recordings are still available. Maybe there was a firmware update. HDD is a WD My Passport 0740 1003 (1000 GB). Tablo firmware 2.2.54, Model 3921RW, Application Version 0.9.0-737366, Roku OS 14.1.4. Have not recorded on the HDD yet but available storage went from almost full to 99%. I will update if there is any problem, but for now, I am hopeful.
When I added an external drive, all my internal storage recordings remained and were expressible. I could watch them and the functioned perfectly, but the Tablo would not record anything new to the internal storage (not that I needed it). All new recordings were saved to the new 2TB SSD.
My understanding was that was the norm for the Gen 4.
That is indeed the norm.