This may not be the correct spot to ask, but here it goes.

I'm trying to eliminate Calibre-Companion since it is abandon-ware.

I bought a new SD card for my Likebooks Mars.

I prefer Moonreader+ over the built-in reader for how it organizes my books.

I connected my likebook-mars via cable to calibre on my computer. I can transfer books to SDcard/books just fine.
Bouye's built-in can find them without any problem. Moonreader+ can't even see them, much less import them. Everything is in epub format.

I also removed the card and tried transferring to the card when directly plugged into the computer. Again Mars was fine. Moonreader is not. Uninstalled and reinstalled it to see if it makes a difference, but no.

Any idea what might be going on with moonreader?
I use the Mars built-in reader for PDFs (it auto-crops, brightness & contrast) and KOReader for epubs because the built in reader is a bit rubbish for books.

The KOReader was downloaded from git (ARM apk) by the Mars browser and has been updated inside KOReader a few times. KOReader can also somewhat use the metadata file that Calibre puts on the Mars.

No problem with KOReader and Calibre, once I figured out Mars's MTP in Linux (you have to stop the file manager system automounting it).

KOReader has a little learning curve but is very good. Wasn't impressed with Moonreader on my Tablet, but I use Lithium (epubs) and Xodo (PDFs) on it. The KOReader is better suited for eink and I use it also on Kobo Elipsa for PDFs only (auto crop).
Why? It works.
But I wouldnt buy it.
I finally got moonreader+ to work
That's more than I was able to do. I'm sticking for the most part with Librera which does a decent job. How about giving a run down of what was causing this Moonreader issue and how you solved it?
I used my Mars to re-format the sd card.

Create a folder on SD card called "books" all lower case. Also used my mars to do that. All lowercase allows the likebook default reader to also see and access the sd card. If you choose "Books" instead, the built-in reader can't see it.

Within Moonreader, It then came up "choose root of ext sdcard to grant permission" when trying to change its default directory to the sd card folder.

I choose the books directory(which didn't work), then the sd card itself (which didn't work), them I chose the books directory again and it worked.

Since I was experimenting with as old card, I put my new one in and did the same thing and it works.
Thanks, I've made a note of that in case I decide to try MoonReader again in the future.
Une fois les libres copiés sur la carte SD (ou dans le répertoire "Books"), il faut importer les livres à partir de moonReader en définissant au préalable le répertoire où ils ont été copiés.