This has puzzled me for far too long. Sometimes, a simple copy/paste of the contents of the iso is enough to make a bootable usb for the OS I try to use, while sometimes, copy paste does not work while other software like unetbootin work. I keep my usb drive in FAT32.
My question is why does this happen and why is it not reliable to just copy the iso's contents? In contrast, dd also always works.