HFS+-fs warning: Filesystem was not cleanly unmounted, running fsck.hfsplus is recommended. mounting read-only.
The kernel is telling you that the filesystem on the external HDD may be corrupted, because it was not cleanly ("properly") unmounted the last time it was used.
It recommends that you run fsck.hfsplus
on the disk to try to fix the problem. Please see man fsck
for more information.
Meanwhile it is mounting the disk as read-only, so you cannot write to it and further corrupt the filesystem before you have had a chance to fix it.