Cannot boot because missing external disk

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

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