While trying to copy/modify a file on a Mac filesystem (hfs+) from Ubuntu, you will get an Read-only file system error (whether you try to do so as sudo or not). To fix it, you need to install hfsprogs:
sudo aptitude install hfsprogs
You then have to mount your partition/drive. With the partition on /dev/sda2 and the mount point on /mnt, it would be:
sudo mount -t hfsplus -o force,rw /dev/sda2 /mnt