Notice: If you don’t know what this is about, then you probably don’t want to do it.
To see all your .wdmc directories the command is:
find / -name ".wdmc"
To delete all your .wdmc thumbnail directories the command is:
find / -type d -name ".wdmc" -printf '"%p"\n' | xargs rm -rf
I keep forgetting the command string so this is my “Note to self”