Latitude 5400 BIOS ver.1.5.1 booting from external USB?

@ArthurMorgan27  on Latitude systems beginning with the Latitude xx90 models, which were the predecessors to the xx00 models (Dell just restarted the cycle after reaching 90), Legacy boot became available only for USB devices (or maybe only USB flash drives rather than fixed disk class devices like HDDs and SSDs), whereas internal storage required UEFI booting.  Legacy boot might have been removed entirely for the xx00 models, possibly in preparation for the fact that Intel has announced that their CPUs will stop supporting Legacy boot around 2021 anyway -- so it's time to step up to a UEFI-based installation.  Do you actually still need to use that bootable SSD on systems that don't support UEFI mode at all?  Otherwise, if everything you have supports UEFI booting now, then Microsoft provides an in-place conversion tool called MBR2GPT that should be able to take switch your MBR disk set up for Legacy BIOS booting and turn it into a GPT disk set up for UEFI booting.

On a side note, how are you managing to boot Windows from a USB device?  Windows specifically doesn't support being booted from a USB device except for Windows To Go installations, but those involve special preparation, USB devices, and licensing.  It's also only available with Enterprise and Education editions, but you say you have Pro.

@ArthurMorgan27  in addition to my reply above, this alternative would be a bit more involved, but have you considered either running a personal Windows environment as a VM on your work system or (if your work IT will allow this) running a work VM on a personal laptop?  Obviously it requires more system resources and getting physical USB devices into the VM can be tricky, but that may or may not be an actual issue for you.  I have a work laptop but when I travel and want my personal laptop with me, I take my personal laptop that contains a work VM that was built from my employer's corporate system image, so I use that on the road.  Works great.

Also, if your existing setup uses Windows To Go under the hood, even if created by some hacked/unlicensed method, then you should be aware that Microsoft is no longer developing Windows To Go because it can't install feature updates to new releases of Windows 10 (source), and according to Microsoft's Windows 10 support model, when the release of Windows 10 you're currently running becomes more than two versions behind the current release, it will stop receiving security updates.  So you might be forced to find another solution anyway sooner rather than later.

Hi, @jphughan :

Thanks very much for your reply.

Let me start with telling you how I was running my personal Windows 10 using my work laptop on an external hard drive. The hard drive is a Samsung 256GB SSD, inside a SATA caddy and with USD 3.0 connection.

I downloaded windows 10 pro .iso image file from Microsoft and used the paid version of WIN2USB software which essentially created a Windows10 Pro, "under the hood" -- quoting your words. This is the best solution for me that I can use only one laptop on the road and separate my personal use and the company issued laptop OS, with has a lot of monitoring and other software running.

The above method requires BIOS to be set to legacy mode to be able to select the USB storage device to boot from, but this newest issue of a company laptop, (latitude 5400) does not have that option available in the BIOS settings, it does not show anywhere to have the UEFI/LEGACY options.

running a virtual machine is not possible in my work laptop's OS, so that's a no for me, do you think if there will be any other solutions? is installing a dual OS a solution for me? if yes, how to do that? 

@ArthurMorgan27  ok I thought that might have been the tool you were using.  As far as I can tell, WinToUSB involves making Windows To Go environments from installations that aren't licensed for it. That to my understanding is a violation of Microsoft's licensing policy and would therefore be illegal.  The fact that WinToUSB itself is a paid application or that the system works from a technical standpoint wouldn't change that.  If Microsoft intended Windows to be used that way, they would offer a means to create this themselves -- which they did with Windows To Go, while continuing to prevent booting Windows from USB devices in all other cases.

But to answer your question, if WinToUSB can't create a USB device that can be booted in UEFI mode, then you're stuck -- although I'd be surprised if that was the case given that UEFI booting has been the default since Windows 8, so they've had plenty of time to implement support for this, and if they don't do it soon then they're going to be useless on modern PCs for this exact reason you've discovered.  So are you sure that WinToUSB can't create a UEFI-bootable environment even today?  But then you'll still be stuck with the problem that Microsoft themselves are no longer developing Windows To Go as a platform, so even getting this working in UEFI mode would be a temporary solution.

In terms of alternatives, I would strongly recommend against trying to set up dual boot on a corporate managed system, because at that point both environments are relying on a common instance of Windows Boot Manager on that disk, and you never know what your IT Department might change through their own policies and tools that might break your environment -- or for that matter what might ever happen in your personal environment that might break your work setup.  If you do decide to go down this road, then at a bare minimum I would repurpose your current USB SSD to store full disk image backups of your work system so that if something does go wrong, you'd be able to restore your system from one of those backups.  Hopefully you're already backing up the data on your USB SSD somehow (and if not, then you absolutely should even if you can find a way to make your current setup continue working), but a dual boot setup makes it even more important to have backups, specifically full disk image backups rather than just your files.

If you decide against dual booting due to concerns or limitations around risk, technical understanding, admin access, and/or corporate policy, and running a VM isn't an option, then I can't think of any other option to create a fully independent but also fully portable Windows installation for you to run.

Thank you very much for your kind response and technical expertise. I have come to realize that maybe it's time to do it in a proper way, and maybe just buy a very portable laptop.

Thank you.