I have two internal SSDs in my computer, one that currently has Ubuntu 20 installed with the following partitioning:
nvme0n1
│
├─nvme0n1p1
│ vfat EFI
├─nvme0n1p2
│ ext4 /boot
└─nvme0n1p3
crypto LUKS
└─luks-long_uuid
LVM2_m
├─vgubuntu-root
│ ext4 /
└─vgubuntu-swap_1
swap swap
That work's well with the default bootloader GRUB2
But I want to install a second Ubuntu 20 on my other drive with a detached boot partition (on an usb thumb). The second drive has the following partitioning :
sda
├─sda1 empty (I will use that later)
│
├─sda2 empty (I will use that later)
│
├─sda3 empty (I will use that later)
│
└─sda5
crypto LUKS
└─sda5_crypt
ext4 /
And I installed my boot partition on my USB as follows :
sdc
├─sdc1
│ vfat EFI
├─sdc2
│ ext4 /boot
└─sdc3
ext4 empty
When I install my second ubuntu with ubiquity from a live CD all works well (I can boot with my usb to my SSD) but when I update grub on the previous SSD (nvme0n1) because the previous configuration has been removed by the fresh installation I cannot boot anymore with my usb (but I can boot to the first SSD/nvme0n1)
I tried to install the second ubuntu with or without an EFI partition, with or without a bootloader. But the main point is that when I update grub from the first SSD/nvme0n1 it's not detecting the second ubuntu (os-prober does not find my second OS even if I mounted it)
The ideal situation for me would be to have my EFI and boot partitions on my usb with the rest of my second SSD in my computer, but I could also go for only my boot partition on a USB.
esp
flag from the EFI partition on my first SSD and install grub normally on my second one (with boot and EFI partitions on my USB). Note there is also no problem withupdate-grub
on all disk taken individually