Sharing my experience re installing ubuntu over my existing os. I normally do this without any issue but my previous installation with Ubuntu 25.10 I was stuck during the manual disk partition step.
Normally I’d just boot on a live usb drive, run Ubuntu installer and perform manual disk partition to re use my existing disk’s partitions.
Here’s how my disk partition looks like :
- 1 GB for /boot/efi (FAT32)
- 4 GB for /swap
- 100 GB for / (ext4)
- 400 GB for /home (ext4)
I ran into an issue where I can’t select the disk (/nvme01) as the boot disk (it’s grayed out). I looked for a solution online (as I normally do) and it turns out my boot partition (1 GB partition in my case) needs to be flag as ESP (stands for EFI System Partition). The option wasn’t available in the installer’s partition step. I had to use gparted which was included in the live usb and set the partition to ESP (don’t set any other flags).
After that the disk can be selected and off I go with the rest of the installation.
If you ran into something like this try formatting your boot partition to FAT32 and set flag to only ESP.
Hope this helps.