Will the drive be disconnected from the server and not accessible by network users during the MBR to GPT conversion process? If yes, then approximately how long will it take to convert a 2.21TB drive that is full of data?
Is this a single disk server or in a RAID formation and is it a hardware RAId or software based?
I would think the RAID would have to be broken so that you can convert all the disks to GPT and then rebuilt.
Note, unless your system is a 64 bit UEFI based one, it is not a good idea to convert the OS drive to GPT as Windows will not natively have installed the correct drivers. There is a way around this but it requires editing the registry.
I'm not sure if the drive would be takne off line, but I suspect it would.
I converted a 2 TB non OS drive a little while ago and I think it took less than a minute.