Mac – Installing Ubuntu 20.04 on external drive with iMac running macOS + Windows 10

bootcampdual-bootexternal-diskmacunix

I have a late 2012 iMac running macOS High Sierra (10.13.6) and Windows 10 (Build 18362) through Bootcamp in legacy BIOS mode and I'd like to do a full install of Ubuntu 20.04 to this external drive and be able to boot it from the iMac when it's plugged in. I've seen guides that address installing Ubuntu either to a Mac + Windows system or to an external drive but not both at the same time. My guess is that the process is similar to this guide but if it's not, how would I go about doing this?

Disks

diskutil list (with external drive plugged in)

/dev/disk0 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *3.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:          Apple_CoreStorage Mac OS X                1.4 TB     disk0s2
   3:                 Apple_Boot Boot OS X               650.0 MB   disk0s3
   4:       Microsoft Basic Data BOOTCAMP                824.0 GB   disk0s4
   5:          Apple_CoreStorage Mac OS X                801.4 GB   disk0s5
   6:                 Apple_Boot Boot OS X               134.2 MB   disk0s6

/dev/disk1 (internal, virtual):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                  Apple_HFS Macintosh HD           +2.2 TB     disk1
                                 Logical Volume on disk0s2, disk0s5
                                 F3382795-876E-40E2-9F63-D1174B9325BF
                                 Unencrypted

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *5.0 TB     disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Untitled                5.0 TB     disk2s2

Internal Drive

sudo gpt -r show /dev/disk0

gpt show: /dev/disk0: Suspicious MBR at sector 0
       start        size  index  contents
           0           1         MBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
          40      409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      409640  2683911088      2  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
  2684320728     1269536      3  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
  2685590264        1288         
  2685591552  1609371648      4  GPT part - EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
  4294963200        2040         
  4294965240  1565305744      5  GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
  5860270984      262144      6  GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
  5860533128           7         
  5860533135          32         Sec GPT table
  5860533167           1         Sec GPT header

sudo fdisk /dev/disk0

Disk: /dev/disk0    geometry: 97451/255/63 [1565565872 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
------------------------------------------------------------------------
 1: EE 1023 254  63 - 1023 254  63 [         1 -     409639] <Unknown ID>
 2: AC 1023 254  63 - 1023 254  63 [    409640 - 2683911088] <Unknown ID>
 3: AB 1023 254  63 - 1023 254  63 [2684320728 -    1269536] Darwin Boot 
*4: 0C 1023 254  63 - 1023 254  63 [2685591552 - 1609371648] Win95 FAT32L

External Drive

sudo gpt -r show /dev/disk2

       start        size  index  contents
           0           1         PMBR
           1           1         Pri GPT header
           2          32         Pri GPT table
          34           6         
          40      409600      1  GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B
      409640  9766869344      2  GPT part - 48465300-0000-11AA-AA11-00306543ECAC
  9767278984      262150         
  9767541134          32         Sec GPT table
  9767541166           1         Sec GPT header

sudo fdisk /dev/disk2

Disk: /dev/disk2    geometry: 73302/255/63 [1177606575 sectors]
Signature: 0xAA55
         Starting       Ending
 #: id  cyl  hd sec -  cyl  hd sec [     start -       size]
------------------------------------------------------------------------
 1: EE 1023 254  63 - 1023 254  63 [         1 - 4294967294] <Unknown ID>
 2: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 3: 00    0   0   0 -    0   0   0 [         0 -          0] unused      
 4: 00    0   0   0 -    0   0   0 [         0 -          0] unused

Best Answer

You should be able to install to an external drive. There are two popular methods to install Ubuntu on your Mac's external drive. The first one is to use default Ubuntu installer option where you use the entire drive. This method does not use rEFInd, unetbootin or the ubiquity -b option. Basically, you use the Mac Startup Manager, Etcher, and Grub. The second method is the one described in your link.

If you do not what to use the entire external drive for Ubuntu, then both methods will require you to partition the external drive. Ubuntu requires at least an EFI partition and a partition for Ubuntu.

Related Question