install Ubuntu16.04 on a removable hard disk, no matter how you use it under window, but you can"t read it by pressing option to boot under Mac.

1. I installed Ubuntu16.04-desktop on a 500g removable hard drive.
2. the installation method was originally boot-1gb swap-16G / 100g
3. Boot boot or other things under windows10, there is no problem with how to use it. If you boot, you will enter Ubuntu by directly selecting removable hard disk to boot.
4. but under Mac, the mobile hard drive of Ubuntu can not be seen by pressing the option button on boot, but in the MacOS system, the existence of the hard disk can be read, although it will indicate that the mobile hard disk is not available.
5. during the attempt, the Ubuntu system is not partitioned, and the whole mobile hard disk is directly selected to install Ubuntu.. But it can never be used under Mac.

ask the boss for help!