I recently came across some old software that was provided as segmented DMG files, aka Split Disk Utility images.
The files were names Disk_Image.001.dmg, Disk_Image.002.dmg, Disk_Image.003.dmg and so on.
Running on macOS Monterey, it seems that the GUI no longer knows how to deal with these ancient split images, double-clicking on the first one to mount it didn’t work any more.
I then thought that I could cat the 001, 002, 003 files together, but that didn’t result in a usable disk image either.
I eventually found a solution with the trusty ol’ hdiutil command.
Running it on the first disk image lead to hdiutil automatically joining them all together and spitting out a new disk image that I could successfully mount and extract the data from.
hdiutil convert Disk_Image.001.dmg -format UDRO -o Disk\ Image.dmg
This gave me a file called Disk Image.dmg that I could then double-click on in the Finder and it mounted as expected.