IIRC, the FAT filesystem (before Windows 95) allowed lowercase letters, but there's a layer in the filesystem driver that converted everything to uppercase, e.g

IIRC, the FAT filesystem (before Windows 95) allowed lowercase letters, but there's a layer in the filesystem driver that converted everything to uppercase, e.g. if you did the command "more readme.txt", the more command would ask the filesystem for "readme.txt" and it would search for "README.TXT" in the file allocation table. I think I once hex-edited the FA-table to change a filename to have a lowercase name (or maybe it was disk corruption), trying to delete that file didn't work because it would be trying to delete "FOO", and couldn't find it because the file was named "FOo".