title_body: Direct copying of epub files on the Kobo internal file system
upvoted_answer: In a nutshell
  
  One way to make visible epub files copied directly on the Kobo file system is to use the Kobo browser once afterwards to download an epub file from the web.
  
  Context
  
  I did some more experimenting on my own, and finally succeeded, though
  I did not explore the precise boundaries of what can work and what will
  not work. I am giving details, so that you know my experimental
  evidence. This is personnal experiment, not anything I read anywhere. So, get second confirmation if that info is critical for your purpose.
  
  If you are only interested in the result, and the technique to transfer
  ebooks to your Kobo without any specific software, simply jump to the
  last section od this answer.
  
  My question was motivated by the fact that I wanted to transfer ebooks
  from my Linux computer to my Kobo eReader. I am running Calibre, which
  I use to improve some of the ebooks, but for some reason that I have
  not yet identified, it will not detect my Kobo when it is connected to the computer
  by a USB cable.
  
  So having read in various places that a Kobo can be seen as a USB
  drive, on which ebooks can be copied, I tried as reported above without
  success, but no indication of the reason(s) for failure. The files
  were simply ignored, wherever I put them. Performing a sync did not
  help.
  
  The experiments
  
  Then serendipity came to my help. I had been wondering for some time
  how I could connect to book sources on the web. I did not know how to
  browse the web with the Kobo. And then I discovered the hidden web browser,
  which they consider experimental.
  In Kobo's own words:
  
  
    Since the browser is still a beta feature, some websites may not
    display properly. Please note that the browser is not officially
    supported by Kobo.
  
  
  Indeed, there is no indication in their user guide (which I had read)
  of the very exixtence of that browser, and I had to search the web on
  my computer to find it. Then I found the help page that explains how to activate this browser.
  
  So I tried the Kobo browser to download a (DRM-)free book from the
  web. It worked ... and (nearly) all the books I had copied by hand on
  the Kobo file system became visible and readable. I am saying nearly
  all, because it actually saw only the files with an .epub
  suffix. Those files had been copied
  
  
  directly in the root directory of the Kobo file system,
  or in a directory codex I had created at the root of the Kobo file
  system,
  or in a subdirectory of that codex directory.
  
  
  I did not check how deep the hierarchy can be.
  
  Note that the book I downloaded from the web was also stored by the Kobo eReader browser in the
  root directory. However, books with DRM are clearly stored somewhere else.
  
  I did not (dare to) check what happens when a file has a .epub suffix,
  but is not an epub file. But I did check that ebooks placed in other
  locations are not seen (of course, I checked only some locations), unless it is a user
  created directory hierarchy placed in the root directory.
  
  I added some more books to check for other means to make then visible to the user. Since downloading a book seemed critical, I was wondering whether a sync induced download would do the job (it does not).
  Then I added two Kobo (DRM-)free books to my account on Kobobooks.com, and
  checked in my computer that they were indeed added to my account on
  kobobooks.com.
  
  Then I performed a sync on my Kobo, and it did download the two
  books. But that did not make visible the books I had previously added
  directly in the file system. It is  worth noting that, though they
  were (DRM-)free, these ebooks were not downloaded at the root of the file
  system, like the epub files downloaded by the browser.
  
  But when I downloaded an ebook from project Gutenberg, the new books I had
  copied directly in the file system became visible in the book collection of the Kobo eReader.
  
  This is already longer than it should. So, I am skipping some details to give my conclusions, for what I understand.
  
  Some basic know-how for adding (DRM-)free ebooks directly to the Kobo memory.
  
  The key idea is that the Kobo eReader (or at least the Kobo Glo HD
  that I use - but I suspect several of them work the same) has a web
  browser, which can be used to download books in your Kobo.
  
  The browser is somewhat hidden, as Kobo considers it an experimental
  item, not officially supported.
  
  The browser download the epub files right in the root directory. And
  then calls some piece of software to integrate it into its
  organization of the ebooks collection. The nice point is that it checks all the non-standard (i.e. non system) files and
  directories that are present at the root level, so as to recreate the information needed
  to integrate them. Aa a result, it takes into account the fact that
  some files may have been removed, modified or moved into a different
  subdirectory.
  
  The only bad news is that (afaik) the only way to get that software
  executed is to download an epub file from some site with the Kobo
  browser.
  
  Hence, copying ebooks from your computer onto the Kobo, assuming
  the computer does not have a web server, can be achieved by connecting
  the Kobo to the computer with a USB cable. The Kobo is then seen as a
  memory device, with a file system which can be mounted as usual (for
  example on directory /mnt with : mount /dev/sdc /mnt depending on where it shows up, here on /dev/sdc --
  note that it is not /dev/sdc1). Then you can copy epub files or file
  hierarchies at the root /mnt of this file system. Then you dismount the file system with umount /mnt, and make sure copying is terminated (Unix sync command) before removing the USB cable, if you will.
  Finally, you use
  the Kobo browser to download at least one extra ebook (possibly always
  the same one :-).
  
  This was described for a Linux system, but should be adaptable to any operating system.
  
  An important point is that all the epub files must have the .epub suffix.
  
  Another thing that I have not been able to determine is whether these
  files can benefit from other services, such as cloud storage. But
  I am very moderately interested in the cloud anyway.
  
  Now, some other user may have better, simpler ways of achieving the
  same result, or more, without using specific software such as Calibre. I find it a
  bit frustating not to be able to used Calibre because it will not
  detect the Kobo USB connection on my machine. I would not mind using a degraded
  version where I would have to tell Calibre when the Kobo is connected and where.