Installing on external hard drive - partition issues

[candydoudou]

Hello,
I wish to install Ubuntu on a new external hard drive, which I formatted like this with OS X El Capitan disk utility (disk3s2 and disk3s3 are in exFAT) :

/dev/disk3 (external, physical):    #:        TYPE                                NAME                              SIZE          IDENTIFIER
                                    0:      GUID_partition_scheme                                                   *1.0 TB       disk3
                                    1:      EFI                                   EFI                               209.7 MB      disk3s1
                                    2:      Microsoft Basic Data                  Ubuntu                            499.9 GB      disk3s2
                                    3:      Microsoft Basic Data                  Windows                           250.0 GB      disk3s3
                                    4:      Apple_HFS                             OS X                              249.7 GB      disk3s4

I created the following partitions :



And after I clicked "install now", I got this message :



After following the instructions, I still got the same message. I re-formatted the partition in FAT32 with OS X disk utility, but I still get the same message.
How can I solve this problem ? Thank you in advance.

[oldfred]

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 external 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 external 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.

[candydoudou]

Thank you for your answer.
I initially wanted to directly make a triple boot on my computer, rather than try making a bootable hard drive first... In order to make things easier, I'll just directly make my triple boot then, and try installing on an external hard drive later then.