I downloaded a USB disk image (.iso file) of 12.04 Precise and followed these instructions to create a Mac OSX bootable disk (.dmg file). The flash drive is enormous (~64 GB), and contained many other things, which I fear have been erased. In the Mac file browser ("Finder"), the USB drive doesn't even show up, as if there is no filesystem mounted.
Can somebody confirm that this process wipes the drive clean before creating the disk image? Is the previous filesystem permanently gone?