That is indeed the case. Simply copying over OS9 bits won't do the job. The appropriate HD drivers have to be installed, too.
Boot from an OS X disc, as suggested. Then either wipe out the OS9 stuff you copied over (if you want to terminate your original experiment), or install OS9 drivers and retry your experiment. Since you stated a desire to preserve your original data, you probably don't want to install OS9 on this drive, because you'll have to reformat it.