Note, Microsoft have provided a tool that may allow you to convert from legacy to EFI boot drives at https://docs.microsoft.com/en-us/windows/deployment/mbr-to-gpt. As with all boot drive transition utilities, please ensure you backup your important data or entire OS first before proceeding with any modifications to your disk structure.
Problem if staying with legacy boot mode: When expanding an SSD or HDD boot drive in legacy mode, if the total size of the FuzeDrive exceeds 2TB, even though the FuzeDrive will present a disk that is > 2TB, you will be unable to expand the C: boot volume beyond the 2TB limit. As pointed out above, this is not a limitation with FuzeDrive but with Microsoft legacy boot disks. The same would occur for driver based RAID disks also.
Example 1: If we have a 2TB legacy boot HDD that was being completely used, and you wish to add a 512GB SSD to it to create a FuzeDrive, the boot volume cannot be expanded to take advantage of the additional capacity added as the boot volume is already at it's limit regardless of the FuzeDrive license limit. The additional capacity can however be used as a D: or other letter drive that will also benefit from the FuzeDrive tiering.
While there are lengthy unsupported procedures online to convert the boot drive from legacy to UEFI boot mode, it is usually safer to reinstall the OS as an UEFI boot volume, the convert to a FuzeDrive. As UEFI allows volumes > 2TB, you will then be able to take full advantage of the additional capacity added to the boot drive.
Example 2: If we have a 1TB legacy boot HDD and adding a 256GB SSD, we can transform the HDD AND expand it to 1.25TB as it is still under the 2TB limit.