after erase, you first reset the board and check again.
I reset the board, and result is the same.
I got a AR150 and 6416 and checked again. I flash ddwrt and it boot successfully.
I then enter uboot console and use the command I give. I only erase 64k so that should be quick.
for 6416, try: erase 0x9f020000 +0x10000
For AR150, try: erase 0x9f040000 +0x10000
After erase, md shows the same but after "reset" (just type reset in uboot but I don't think it is different from power cycle), md shows only FF, which means the flash is erased.So everything is normal in my side.
But I do remember I have similar experience with ddwrt. I just tried to erase the ddwrt firmware several times and I can revert back to our own firmware.
So I think you may change a power source (who knows if this helps) and try to erase. If not OK, do it several times.
I have revert file for AR-150 (reverts from ddwrt to GLI firmware 2.24) that LOM (ddwrt forum) made for me. Not sure if it works on 6416…
revert_file_on_my_gdrive