You are using Ubuntu or any other linux dist, and have an external drive. You HAVE to format it with HFS+ because you want to use it with a Mac. Grrr...
No panic.. You are able to format your external drive using Ubuntu.. First thing to do is that you have to install "hfsprogs" package. Simply install it by typing the following in a terminal:
sudo apt-get install hfsprogs
Then, you can just use GParted to format your drive. Go to System > Administration > Gparted, select your external drive from the dropdown menu on the right. When you choose to format your disk/partition, you'll see "HFS+" as an option. Select it and wait a little bit. And.. Da daaa.. Now you can use your external drive with a Mac!
Hope it helps!.. Enjoy..