Calibre can not send book to Kindle

If you're experiencing issues with your Kindle mounted as read-only it is probably due to some errors in the file system.

Find the folder of your device typing mount

In my case the output is:

/dev/sdc1 on /media/Kindle type vfat (rw,nosuid,nodev,uid=1000,gid=1000,shortname=mixed,dmask=0077,utf8=1,showexec,flush,uhelper=udisks)

meaning that I have to correct the filesystem on /dev/sdc1

You can do that using fsck: sudo fsck.vfat -r /dev/sdc1

If you get ask which partition you want to correct select the first by pressing 1 and continue.

Unmount your Kindle and remove and plug again the USB cable, you should now be able to send and copy files from calibre to your device.

After running the fsck.vfat command, I was told that the dirty bit is set, which was probably because of unclean unmount. First I accepted to clear that bit, then it offered to correct the blocks in the first partition, which I declined. After reconnecting Kindle back, it mounted cleanly as RW partition. Thanks for the tip!