Scenario: you want to migrate your system from a drive smaller than 2 TiB to one bigger than 2 TiB, and the program doesn’t let you
This happens if the old drive is partitioned as MBR and not as GPT. The maximum addressable space for the MBR layout is 2 TiB. The copy must have the same partitioning layout as the original, but drives bigger than 2 TiB can’t be partitioned as MBR (it’s technically possible, but only the first 2 TiB would be addressable). That’s why copying is refused; it’s an unfortunate limitation caused by the shift in technology.
If the source drive is only used for data storage, you can convert it to GPT before copying. Alternatively, you can initialise the destination drive as GPT and copy partitions one-by-one, resizing as necessary.
If the source is a bootable operating system drive, conversion alone is insufficient. Booting Windows from GPT is very different from booting from MBR. It requires a UEFI-based BIOS, a special EFI system partition on the drive, different Windows boot files, and a special record in the BIOS NVRAM. These can only be created when doing a new Windows installation. If your BIOS is EFI-compliant, here’s how you can migrate.
After these operations, the new system should be functional.