TL;DR If the writing speed on external HDD (or flashdrive) is too slow try this:

  • Open file /etc/usbmount/usbmount.conf
  • Search the line MOUNTOPTIONS="sync,noexec,nodev,noatime,nodiratime,uid=1000,gid=1000"
  • Remove the sync so the line must be MOUNTOPTIONS="noexec,nodev,noatime,nodiratime,uid=1000,gid=1000"
  • Save and reboot

Then writing speed should be increased a lot.

If you want test the actual writing speed, supposing your HDD is mounted in /media/usb0, use dd if=/dev/zero of=/media/usb0/output bs=8k count=10k