"Save to Image: Read Only" is greyed out
 

I try to create an image of a disk, the first two choices are available (sparseimage, sparsebundle) but the choice that I want is greyed out (read only). I can't figure out why.

I'm still running High Sierra. The source is an external HDD (APFS). I tried two different destinations and neither worked (an external APFS HDD; an internal HDD HFS+ volume that's not the startup volume).

If anyone wants to know why I want to create read only image, it's because I want to create a snapshot of my disk when I feel it's entirely problem free and store the image as an archive. I also have a separate bootable clone that I smart update regularly but sometimes problems don't show up immediately and I might have backed up a problematic drive to my clone so I would have no more good backup. Saving an archive image periodically is an extra safety measure. As for why a read only image instead of a sparsebundle, it's because read only image can have High Compression whereas a sparsebundle takes up the same size as the source disk's data.

If there are better ways to create archive image please let me know.

APFS cannot be stored on a read-only image; conversion fails when attempted.