My guess is Windows is the culprit here. It is likely stripping the resource fork off, or otherwise altering the disk image file so Disk Copy will not recognize it. When you re-save it in mini vMac, Disk Copy is adding back the attributes Windows likely stripped. This is why I try to keep my files in the same environment. When I download disk images, I usually do it inside something like Basilisk II so the image is downloaded into a Mac OS of similar vintage, then copy the disk image into a mounted disk image which I can then transfer via zip to any other Mac. Direct transfers to a 128K and 512K are achieved via Mac Terminal and null modem within the same emulator.

But good work around! Thanks for the tip.