I have an iMac (model 18,2) running Mojave. I have installed a new external (USB 3.1) SSD, and installed Catalina on it (and am in the process of migrating to it as the primary load source).
When I am running under Catalina, both the new SSD and the internal hard drive show up as single volumes ("SanDisk SSD" and "Macintosh HD"). But when I am running under Mojave, the SSD shows up as two volumes: "SanDisk SSD" and "SanDisk SSD - Data." Moreover, when I put custom icons on those two volumes (via Get Info) from Mojave, then reboot to Catalina, it's the icon I applied to "SanDisk SSD - Data" that appears.
This is most confusing. Can somebody shed any light on what's going on?
(FWIW, my customary disk drive icons are based on a photograph of a 1960s-vintage removable disk pack, of the sort that might go into a "Merlin" drive on an IBM 370 or compatible.)