So you’ve decided to install Ubuntu Budgie on a separate drive and prepare for Dual Boot.
You install the OS, setup the install location for the bootloader on the disk dedicated to the Ubuntu Budgie, only to find that the /EFI partition on the other disk was used for bootloader installation, regardless of your choice.
The main problem steams from, that even in case you have for example separate disk dedicated to the EFI, Installer assumes that you want to install boot files for Ubuntu to it instead on the specifically made EFI partition.
To make sure that the EFI partition that you make during the install is used correctly we need to use Gparted.
Open the Gparted and find the efi partition used by the main drive. Edit the partition by removing ESP flag. Execute operation.
Open installer and setup your install scheme. After you are finished with installation, do not restart. Go back to Gparted and modify EFI partition back to its default state. Tick the ESP option to return boot flags to EFI partition used by other OS.
With this, you can install OS on separate drive without worrying that the OS will use partition from other disk or being unable to boot in your Ubuntu Budgie system in case the disk used by main system is not there anymore.