To share disks between a Raspberry Pi and a Mac, you can't use any of the linux-only filesystems, or the OS X only filesystems. Instead you're left with:
vfat: Well-supported everywhere, but the max filesize is 4gb. This can hurt you if you're working on video.
exfat: Relatively well-supported, no max filesize.
Exfat isn't supported in the linux kernel (and therefore the Raspberry Pi) by default, but there is a userspace driver. To install it:
sudo apt-get update
sudo apt-get install exfat-fuse exfat-utils
Now you can mount your external disk as usual:
sudo mkdir /media/mydisk
sudo mount /dev/sda1 /media/mydisk