My Pi was working fine, but I rebooted it, properly from the menu, and it never came back up. Did all of the standard troubleshooting. Moved the card to another Pi (3 B, also), then that Pi didn't boot. Took the card from the second unit, put it in the first unit, and it booted. So, it's a problem with the card. However, putting the card in a reader, the card appears to be fine. No files changed recently. But the card isn't a bootable card.
Card-1 won't boot Unit-1 or Unit-2. Card-2 boots both Units. Card-1 appears to read fine in a reader.
Any leads? Tips to revive it? A utility for this type of card?