$ umount m (finished, so umount it)
$ rm initrd.tmp (no need for initrd.tmp anymore)
$ rmdir m (no need for mount point either)
The initrd.d directory now contains what is a minimal root-lookalike
filesystem, with contents:
bin/ dev/ etc/ linuxrc lost+found/ modules/ proc/ sbin/ tmp/ var/
(Side note 1: This isn't the *entire* contents of the initrd.img ramdisk
file. All of the special device files in the /dev directory could not
be duplicated using a simple copy so, if you want to see the actual
contents of /dev, you'll have to do it while you have the filesystem
image mounted.)
(Side note 2: You might want to poke around the ram disk image, just