The way Valve set up the Deck is that the only storage initialized is the internal SSD and the SD card in big picture mode (home menu) which makes sense if you’re playing on the go - you’re not going to hold an external HDD in your offhand on the plane lol.

I believe using the dock doesn’t have any impact on how the Deck handles drives either, the official FAQ mentions it’s more akin to “plugging an external USB hub”, so that won’t solve the automount issue.