I believe you will need to install the PC Exchange extension for Mac OS to support ProDOS.

The best thing I've found for making IIgs disk from images Mac is DiskDup+ or DiskDupPro... I've found the formatting is much more reliable than Disk Copy. You can download the IIgs system disk images from Apple's Older Software Downloads page (right at the top) and write them to disk with either Disk Copy or DiskDup+.