Running on Arch32, with 'hfsprogs' installed from the AUR.
I've previously had a partition (/dev/sdb1) mounted as ~/Storage, owned by my user, and had no issues.
Now, however, for reasons I cannot comprehend, mounting it both via /etc/fstab (I'm the only user on the system, so no worries about others having it mounted) or mounting it directly while logged in, results in it being marked as a Read-Only filesystem, and I can't seem to change it.
I'd rather not switch off using HFS+, since while it's been slightly slower than EXT3, I've found it's been better for long-term storage (hence the name, Storage)

I don't know how to fix this in Linux but I had a similar problem with my old ipod a number of years ago, I had a power cut and the ipod was not ejected properly after that it only mounted read-only ( it was formatted hfs+) the only thing that fixed it was to mount it in OSX then eject it ( presumably OSX fixed any errors on the disk ), if you can mount your disk on OSX maybe this will work for you?

The problem is, it's not on a Mac, and never has been. I created the HFS+ partition myself as a long-term storage area by using GParted.
I can run fsck.hfsplus on it, and see if that solves anything. I probably should have thought of this before.