Dual boot - External SSD not bootable


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Nov 17, 2023
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NOTE: you can jump direclty to the TLTR - Long story short at the bottom of this text.

Long story:
My laptop decided to die on me, I took the ssd (Samsung) that had dual boot (Windows 10 and Linux Mint) and put it in an usb enclousure. To my surprise the Windows 10 and Linux Mint installations where able to boot just fine on my sister's notebook via usb, I just had to change in the BIOS settings the boot from UEFI to CMS (my old pc had a BIOS and no UEFI).
Since I wanted to avoid changing this BIOS setting everytime I had to start a different system I decided to make the jump and change WIN10 to Win11 and MBR to GPT. I decided to do all the following operations with the disk inside my laptop leaving it the only drive in it just to be sure that the boot partition wasn't installed on another drive. I made a backup of my WIN10 data, an image of the Linux Mint system and then I formated my Samsung SSD to GPT. With Rufus I managed to install Win11 (that has no TPM 2.0 and safeboot available in the system) and then restore my Linux Mint with the image I had made. I had a perfectly working system with dual boot and Grub was managing everything just fine (beside the Win11 and Mint I also had a nice entry that allowed to log directly into UEFI, nice!).
After that I decided to just take the drive, put it back in the usb enclosure and boot the OS but the external SSD is not bootable in this way.
Can I make it bootable without having to reinstall all the operating systems (via the Rufus "Windows to go" option that I discovered only after)?
Can I make some grub changes or something else to make it bootable via usb?

TLTR - Long Story Short:
  • I took a working dual boot (WIN10 + Linux Mint) SSD from a non working laptop and put it in and usb enclosure
  • this usb ssd booted just fine on a different laptop (changing UEFI to CMS)
  • I put this SSD inside the new laptop
  • I formated it to GPT (before was MBR)
  • I installed WIN11 and restored the old Linux Mint via backup, everything worked with GRUB managing the dual boot
  • I put the ssd back in the usb enclouser and now the ssd is not bootable anymore

How can I make the external usb SSD bootable without having to reinstall the operating systems?

The Drive is present in the the UEFI/BIOS and if I press F12 in the Toshiba notebook I can see the drive, I select it and the system ignores it and proceeds to load the internal SSD.

By the way here's the structure of the external SSD:
Diplayed in GParted:

In Disk utility:






Thank you

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