Trying to set an exterior Bootcamp drive as the Startup Disk;

Your external disk has a MBR partition scheme. A Mac can't boot from a MBR disk and needs a GPT partition scheme to be able to boot from; "And to function as a startup disk, it needs to be using a GUID partition map." from support page: How to set up and use an external Mac startup disk - Apple Support

I can't explain why you're able to boot from it with your MacBook Pro, it shouldn't as far as I know...

You started this post in the MacBook Air category. The issue is related to Windows/Bootcamp, I would suggest two options:

1. I can ask the 'host' to move this post to Windows Software / Bootcamp category.
2. You start a new post with the specific issue in Windows Software / Bootcamp and refers to this post.

Experienced users with Windows/Bootcamp will pickup the post more likely in the Bootcamp categoy then here. Let me know what you do/want.

Can you post the output of

bcdedit

bcdboot C:Windows

Also, check in diskpart, if C: is actually assigned to the Windows partition.

There are a few things you can try to resolve the exact issue in your question.

Reset NVRAM

Older Macs had what's called Parameter RAM (PRAM), newer Macs use Non-Volatile Random-Access Memory (NVRAM). It may be worth you resetting this. Here’s how to:

1. Shut down your machine. Yes, a full shut down, not just logging out.
2. Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
3. Hold those keys down until your Mac reboots again and you here the startup chime.
4. Let go of the keys and let your Mac reboot normally.

Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Once you boot in, go to System Preferences > Startup Disk and check to see if you can now select your Windows Bootcamp volume as your startup disk. If you still can't, try the step below.

Use the SDM to change your default Startup Disk

Another way you can select your default startup disk is to use the Startup Disk Manager. Follow these steps:

1. Shut down your Mac
2. Switch it on again and immediately press and hold the option key
3. Release the key when your bootable volumes start appearing
4. Press and hold the control key while you select your Windows Bootcamp volume

Once you've done this, your Windows Bootcamp volume should be the default startup disk. If you still can't, try the step below.

Use the Windows Boot Camp Control Panel

You can also try using the Windows Boot Camp Control Panel instead. Use these steps:

1. Boot into Windows
2. View the Control Panel
3. Select the Boot Camp Control Panel
4. Select the Startup Disk tab
5. Select the Windows volume as your default startup disk

Once you've done this, your Windows Bootcamp volume should be the default startup disk. If you still can't, see other options below.

Other options

Another couple of options are:

• Trying rEFInd
• Using Terminal to set your Startup Disk (if you're comfortable doing so)