That was a message it used to give when the disk it was trying to read was unformatted. Can you INIT the disk first?
OSX users have had a spotty history with Prolific drivers. Hopefully they've improved. You can see some of the stories in the ADTPro forum:
http://sourceforge.net/apps/phpbb/adtpro/index.php
I'm missing some pieces of the story line, I think. You used MacADT to communicate between your LC and IIci machine with a printer cable (null modem). Then, you changed to a Prolific-based, USB-to-DE9 serial adapter. What does the cabling look like now? Is it a null modem too? Do you have the pinout?