Re Mithrandir, The first thing the machine does when you insert an SD card is check on the filesystem type. If it's not to its liking it re-formats it, and it's unlikely to be something the machine cannot read.
I've been playing around a bit more, and am even more puzzled, as a consequence. I tend to use Es File Explorer. I don't particularly like it, but, of the many I've tried it seems to be the least worst. It still ignores the external storage card. I tried a couple more and they likewise ignore it. Casting about, I did find, in the default apps that came with the install, another file manager, with the usual title 'File Manager' and no acknowledgements. The icon for this was seemingly a wooden box, or may have been a filing cabinet drawer, of the old variety with a CD disc and other objects in it. This, oddly did show the external card under the title SanDisk, but showed no other storage, like the normal sdcard title.
Even odder, in populating the Music directory with files FTP'd from my phone, I found that both internal and external storage, as depicted in the two file managers, had the same music files installed. Deleting the ones in SanDisk, so as to know which I was looking at, a few minutes later, the ones in the other (Internal) storage disappeared too. Even odder, looking through other directories, I came across a photo which I knew had not been taken on the Gemini, but on my mobile phone. I've no idea how the thing got on there. The FTP operation was not between the Gemini and the phone. Instead, I'd sent the file to my PC which runs Mageia Linux, and is an FTP server, and taken them from there onto the Gemini. I took another pic with the phone, and a minute or two later, it too appeared on the Gemini.
I've been looking at Sailfish, which I also installed and I'm not impressed. Too much flash and not enough substance. I tend to take a utilitarian stand with computers. It's a tool and requires no embellishment. It's simply there to do a job, and nothing else. So, I've decided to break it all down, and re-flash it with Android and Debian. although I'm not at all happy with Debian either. As an old Unix hand, I'm a bit aghast at having a system where any user can assume root powers simply by typing sudo. It's one of the reasons I never voluntarily use Windows with it's haphazard admin functions. Although I'm the only user on my machines, I am acutely conscious of being either one or the other. I picked Mageia, or Mandrake as it was then, as the nearest I was going to get to a proper system.
Cheers, Rod Goslin