The DISKCOPY42 file suffix is associated with the Apple Disk Copy 4.2. Disk Copy was the default utility for handling disk images in System 7 through Mac OS X 10.2.
How to open:
Your best option to accessing disk image formats is to use some of the virtualization programs that typically support most of the common disk image formats which can be either mounted as virtual drives or burned on physical media.For general information try to look at:Basic info how to open disk images
How to convert:
Most disk image formats can be nowadays converted to standard ISO format, however conversion between various formats is typically possible. Your best option is typically some disk image management tool with broad format support, for example ISOBuster.
For general information try to look at:Basic info how to convert disk images