I added installed a brand new Sandisk Extreme Pro 512GB card but the web interface shows only 119GB available in T-Flash. A search of the forum suggests that 512 should be the available limit and some claim to be running 1T. Suggestions?
Try to format it to compatible file format.
Do you have partition on the TF card?
Would it recognize any storage at all if the formatting was incompatible?
As it turns out, it will recognize the larger card when I actually put that one into the router — instead of the 128 GB card laying next to it that I picked up from the rubble on my desk. 
It would only recognize those partitions with compatible formats, hence showing you a lesser size than the actual one.
I’ve never received a factory new SanDisk card partitioned as you suggest. The problem here wasn’t partitions, it was user error. 