try incerting a little > in there.

cat /dev/cdrom>/mnt/hda3/iso/my.iso

and yes the hdd must be mounted.
hmm, can you mount a CD? if so try mounting a cd and
do a mount|grep cdrom this should give you the proper device.
so should cat /etc/fstab|grep cdrom.
dmesg|grep CD might help aswell.
heh, there are several waqys of finding what your CDrom is called.
and it should work. if not try the dd comand...