[ubuntu] Installing on external hard drive - partition issues

Do not know Mac.

But not sure why error? I looks like it is divisible by 8 (check my math), which is what is important.
Partition does not start on physical sector boundary.
First, understand that most partitioning tools have moved to a policy of aligning partitions on 1 MiB (2048-sector) boundaries as a way of improving performance with some types of arrays and some types of new hard disks (those with 4096-byte physical sectors). See article by srs5694:
http://www.ibm.com/developerworks/li...-sector-disks/
Post on 8-sector boundaries alignment by srs5694
http://ubuntuforums.org/showthread.php?t=1685666
it's 8-sector (4096-byte) alignment
http://ubuntuforums.org/showthread.php?t=1768635

Note that full install to any drive other than sda in UEFI mode has some issues. Grub only installs to the ESP - efi system partition on sda. And you then have to copy files to your install.

And Ubuntu's UEFI grub only installs to the ESP on sda, or not the exteal drive and not to /EFI/Boot/bootx64.efi. For my PC UEFI full install to a flash drive I manually copied /EFI/ubuntu on sda's ESP to flash drive's ESP. Then copied it again to /EFI/Boot and renamed shimx64.efi to bootx64.efi. I then updated fstab to have correct UUID for ESP on exteal drive.

The version of grub in a full install is hard coded to find the rest of grub in /EFI/ubuntu so both copies are required. There are ways to directly install grub as bootx64.efi, but then you have to manually maintain grub.cfg.