Board: Nanopi M4 with 4GB OS: Armbian_5.65_Nanopim4_Debian_stretch_default_4.4.162 Boot from SD, rootfs on an external HDD 8TB (WD MyBook).   I attached another external HDD (Toshiba 2TB). I have been trying rsync -avh from the Toshiba one to the main MyBook one. It ran for under a minute before reporting files were vanished and exited with errors. It appeared the / is gone. nothing return from ls. Hard-reboot, fsck ran, everything's back to normal.    I've been try