You need to start with some other make (any other make) of 8GB SDCard and a copy of plain Raspbian. Get that booted, get the suspect SDCards mounted in a USB reader and take a look at what's happened with them. Start with the logs in /media/pi/*/var/log (which is where the auto-mounter will put things).
Then pick a suspect card, copy plain Raspbian on to it (using the GUI SDCard Copier). Shutdown cleanly, swap cards and see what happens when you boot that.
Grab some big files from http://www.sample-videos.com/
write them to the 32GB SDCard, shutdown, reboot, see what happens.
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