Have you tried seeing how the router is treating the flash card? Please issue "show file system" command on the router. Most likely the router could be treating it as slot (I think the router treats any card less than or equal to 128 MB as a slot and then onwards a disk). Please try "dir slot0:" and see if that helps.
I think you are looking at a bad I/O card. Since you have tried multiple flash cards and none of them are working, I suspect it to be a bad I/O module. I would suggest contacting Cisco and requesting replacement for the I/O module.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...