@Ramhound: Mac OS X only requires a GPT partitioning scheme to INSTALL, it will boot from MBR if your copy the OS to such a disk.
@John: The simple answer is no, you can't. Here's why.
You should not convert a Windows installation in place, you should instead install from scratch. This is one of those problems where you would spend more time making copies of your data, and technical massages of the machine to get it to do what you want than simply reinstalling.
Windows does not care about not boot disks, so you can make your secondary disk into a GPT disk if you want to. You would have to do this if you had a 3TB drive anyway. Now, there are no easy tools to convert in place, you would be wiping the drive clean to change to GPT. (Remember, GPT and MBR are the underlying structure of the partitioning scheme, so by changing them, you are wiping the partition table clean.
Now, you were not clear on your usage of Hyper-V, is your system installed on top of Hyper-v? If so, then the answers change, because your system can be "copied", reinstall Hyper-V onto the GPT disk, then "copy" the system back, and you don't go through the reinstall process.
This is not how most people use there computer, but you mentioning these system makes this a possibility.
Sorry I can't be more positive.
"CSM" and "legacy" are synonymous in this context, so -10 points for clarity to whoever wrote your firmware's user interface. Since you want to install in EFI mode, the "UEFI" option makes the most sense; if the names are truly descriptive of their effects, this will force the computer to boot in EFI mode, which is what you want. The "UEFI or Legacy OS" option might also work, but is more hit-or-miss -- there's really no way to know in which mode the Windows installer will boot. If it boots in EFI/UEFI mode, then the effect will be identical to using the "UEFI" option; but if it boots in BIOS/CSM/legacy mode, you'll be wasting your time. You should definitely avoid the "Legacy" option. A caveat: Many firmware interface options are very poorly named, so you may find that these options don't have the effects that they seem to describe.
I don't know offhand what the Windows installer does when confronted with an MBR disk during an EFI-mode installation. It might work fine, particularly if the disk has no partitions; but if you want to be 100% certain that you're installing in EFI mode, you might want to pre-partition the disk using GPT. Personally, I'd use a Linux live CD for this, like Parted Magic or System Rescue CD; but if you're not comfortable with Linux, you may prefer to use a Windows emergency disc and Windows tools. Either way, ensure that the disk uses GPT and the Windows installer can take care of the rest.
Best Answer
Let's start with some definitions.
As you mentioned laptops and the maximum capacity for 2.5" drives is 1TB currently and your priority appears to be maximizing compatibility, a traditional BIOS/DOS/MBR is recommended.
You may utilize
gdiskor another tool for the conversion (though loss of data is high for such a task) and then install GRUB bootloader to the MBR. You may need to enable BIOS-compatibility mode in the UEFI settings to boot.