I wanted to have write access to my HFS+ external hard drive. It took me (frustrating) hours to find out, since I kept on having it mounted automagically (i.e. seconds after I inserted the USB plug in my labtop) as read-only. So I could read from it, but not write to it.
Write Access to an HFS+ Drive from Ubuntu
This post did the trick for me. In other words, I had to install the hfsprogs Ubuntu package and run:
fsck.hfsplus [block device]
So for me it was:
Nothing wrong was found after running that check, but it apparently needed to be run. Because right after, I unmounted it, and let it mount automagically, and it worked.