Bootable ssd not available in "boot priority"

I wanted to make a portable plug and play ubuntu ssd. I installed ubuntu and it worked on the laptop I used initially. When I plugged it into another laptop, the ssd is not visible in the boot priority or in "this PC", but is visible in disk management and also displays as a usb device in the asus rog BIOS settings.

UEFI FIRMWARE SETTINGS IN THE NEW LAPTOP

LIKE THIS

Disc

Solution 1:

There are a list of reasons this can happen:

disabled boot from external device / 2nd drive / USB boot for security (turn them on if you find something like that)

For best chances I would recommend setting Secure boot off, legacy mode on. If you turn on legacy mode and try to boot it, you might get away without the UEFI nightmare because it will boot the drive just like an older PC.

UEFI as metioned by many already and the proper format and table for it on the drive. It could be that the reason why it only works on one PC is that older ones do not have UEFI restrictions. MBR or GPT table, in this case you are already using MBR so stick to it. You can create it in Gparted device>Create Partition Table (this will delete all partitions). Then to create the EFI partition with Grub and esp+boot flags. After the partition table is made you need to create an approx 500MB fat32 partition which will be your EFI. After creating it you can right click>manage flags and add esp + boot. This is where grub will be installed. I know this process means remaking the whole ssd drive, but all these things are needed for UEFI to recognize it and boot.

(I see the ? in file explorer screenshot. Windows cannot read ext4 filesystem so it does not see the Ubuntu partition contents. This is also the reason why Disk Manager only shows "healthy partition" and not "ext4 partition", it simply cannot read it)