I was inspired by your solution thank you Robert Selfe.
Using an Atomos Ninja Inferno with my GH5 I was frustrated to have to import enormous 10-bit 4K30P ProRes files from the Ninja hard disk when I just needed a tiny fragment of each. So I copied the Panasonic’s SD card (the one it recorded as a backup) across to the Ninja drive but then in the DCIM/109_PANA folder I deleted Panasonic .mov files and replaced them with the Atomos ones.
Lo and behold FCPX 10.3.3 now sees it as a card rather than a drive.
Even though the names were different it worked and I was able to import just sections of these enormous 10-bit 4K ProRes files (and create proxies to work with).
Appalling media management I know (data wranglers look away!) and I may yet come unstuck, but I thought I’d mention it as a workaround for anyone truly tearing their hair out.