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
