On the latest Mac system, you can’t open the dmgpart segmented file, and you can’t select all, and you can open all on the previous system, but now you can’t!
Is this related to DropDMG or are you referring to the Mac Finder? What does “select all” refer to?
I’m not exactly sure what you mean because I don’t think .dmgpart files were openable before. Normally, you would mount a segmented disk image by opening the first part, whose name ends with .dmg.