Rcently needed to mount my hfsplus formatted external disk on my Raspberry Pi. I would get a read-only warning.

mount: warning: ext_disk/ seems to be mounted read-only.

Finally I figured out that doing a fsck.hfsplus on the disk and remounting worked well.

The steps I took:

$ sudo apt-get install hfsplus hfsutils hfsprogs
$ fsck.hfsplus -f /dev/sdaX
$ mount -o force /dev/sdaX /mnt/