Maybe some of these suggestions can help.
Contrary to what you have been told, it is not necessary to make this disk as your first bootable device in bios settings. Instead, if the boot menu option is turned on in bios and this cloned disk is recognized as being a bootable device, you can restart your computer and acccess the boot menu by tapping a keyboard hotkey during the post process. With a Dell it is F12. Other brands use different keys to access the boot menu. Find your device in the following list and use the applicable boot menu access key. Tap it as soon as you see the brand of your system popup during boot..
How to get to your Boot Menu or your BIOS settings? Use different hot-keys that go to these settings.
www.disk-image.com
Once you have the boot menu on the screen select whichever disk you want to boot from. IF YOUR EXTERNAL DRIVE IS NOT ON THE LIST, IT IS NOT RECOGNIZED AS A BOOTABLE DEVICE.
If you still are unable to boot from this ssd, your BIOS boot mode may not compatible with the SSD. If you cloned a GPT hard disk to MBR SSD, or cloned MBR hard disk to GPT SSD. It is necessary to change the boot mode from UEFI to Legacy or from Legacy to UEFI in bios settings before the device will be recognized as bootable. Remember though, this setting would have to be changed back before your system would boot from your current OS system drive.
A corrupted cloning process or the source disk used for cloning may have had bad sectors may also be contributors why the disk won't boot.
Another way to fix the boot partition on the cloned drive is to use DISKPART by booting from Windows installation media. However, without knowing your level of expertise, I won't be the one to recommend this method to you as one can get into a real pickle if used incorrectly.
For so many reasons, I NEVER clone but make an system image of the source system and then restore that image to a target drive. Cloning has just never worked out for me as I've encountered issues that took a lot of unnecessary time and effort to straighten out.
Good luck.