Executing 'lsblk -f' after plugging the external disk would confirm/infirm
you hypothesis. I believe its ext4 filesystem would then be attached at
/media/data, thanks to the last line in your /etc/fstab... but the default
directory, without that line, would be similar (at least it would be in
/media), if not the same. Since you apparently have not added the line
yourself, I guess your external disk was plugged when you installed the
system.
All that written, I do not think the error has any impact on the init time,
because nothing depend on that filesystem being mounted. If you are
interested in graphically analyzing what takes time during the init:
$ systemd-analyze plot > systemd.svg
"systemd.svg" can then be opened with Eye of MATE (or any other viewer that
understands SVG).
