Corrupted SD card after bad unmount.
 

Basic / new user. I've tweaked commands, but mostly use GUI.

I hastily removed an SD card before ejecting.

I then inserted a different SD card, which is now corrupted in all systems and readers, including the original board mounted reader, camera and external USB reader.

What can I use / do to recover the photos from the second SD card?

My user level is that I am able to use the terminal for commands and most any GUI program. Any help is appreciated so I can recover valuable photos from an Opera photo shoot. ;)

1) Use dd to make a backup image of the SD card, eg:
replacing /dev/sdb with the device name of your SD card

When that finishes,
2) Use photorec to scan the backup image for any and all image files. You'll lose filenames, but the creation dates should be retained, so it shouldn't be difficult to rename them to the normal naming sequence once they're recovered.
http://www.cgsecurity.org/wiki/PhotoRec_Step_By_Step

Running fsck should be the first step, before you opt for more radical measures. Make sure you make a backup first as suggested above.