I have thought OSMC makes some sort of format. In such case I have probably performed some action which deleted it, but it seems really strange to me.
My fstab:
/dev/disk/by-id/usb-PRETEC_32GB_13052410440805848869-0:0-part1 /home/osmc/filmovnicek btrfs defaults 0 0
I just copied it from Raspbian where it worked.
The reason was that I mounted it on another place then media and then shared it via Samba.
I have never seen my content, it was the reason why I firstly though about the formating.
Anyway, thanks for replying.