Cannot boot because missing external disk

Question

Using Fedora 24, I had configured in /etc/fstab an external usb drive:

UUID=6826692e-79f4-4423-8467-cef4d5e840c5 /backup/external      ext4    defaults  0 0

When I unplugged the usb disk and reboot, it does not boot

That is the error message:

[ TIME ] Timed out waiting for device dev-disk-by\x2duuid-6826692e\x2d79f4\x2d4423\x2d8467\x2dcef4d5e840c5.device.
[DEPEND] Dependency failed for /backup/external.
[DEPEND] Dependency failed for Local File Systems.
[DEPEND] Dependency failed for Relabel all filesystems, if necessary.
[DEPEND] Dependency failed for Mark the need to relabel after reboot.

Why does not boot? is it a bug? a feature? of systemd? I know that was a mistake of me, I had to set options to "noauto", but anyway Why booting process stops because a non-critical directory of FHS is missing?

Solution

Using the nofail mount option will ignore missing drives during boot. See man pages fstab(5) and mount(8).

nofail Do not report errors for this device if it does not exist.

So your fstab line should instead look like:

UUID=6826692e-79f4-4423-8467-cef4d5e840c5 /backup/external ext4 defaults,nofail 0 0