Before you read any further: If your file does NOT EXACTLY END WITH .001, .002, etc... (e.g. ASD_3S152.001.dmg) then you need to jump to the split Disk Utility disk image section. When you come across split files that end with sequential numbers (e.g. .001, .002, etc...), before you can use the file, you first need to join the split files together into a single, whole file.  A split ZIP file could look like this, for...