2TB external drive, disk initialization: MBR or GPT

GPT will not be usable by XP and earlier systems (I think), so avoid it if that is an issue. Not having seen an XP machine in 3 years it's not an issue for me so I use it in preference to MBR, but it makes little difference at this point in time. At some point, you will have to use GPT, until then, make a choice.