Just out of curiosity, how did you download the disk image and get it into your 170? It's possible that OS X altered the resource fork during the download, or after you tried to mount it under OS X, so that Disk Copy could not recognize it on the 170. Whereas a freshly created file within OS X retained the necessary ID. In particular, I have found that Images mounted under OS X often do not work properly in a 68K environment. This was a known problem under OS 8, when using Disk Copy 6. While DC could make a perfectly usable 400 or 800K disk, if the disk were mounted under the OS 8 Finder, it would be corrupt for use under 68K Finders.