Scenario is that you have external Hard Drive that you were using in Mac OS X as perhaps a Time Machine backup drive and you now want to erase this drive in Windows. But you find that you cannot remove the 200MB EFI System partition. Here is how:1On the command prompt type diskpart and answer to any possible UAC prompt.2On the new diskpart prompt, type list disk. Note the Disk Number column.3 Type, select disk no. with no. being the partition you wish to delete. Usually partition 0 and with those 200 MB of size4 Finish by typing, clean.This should only take a short moment and it will be complete. There is no message letting you know this has been done. Just quit from the command prompt after a moment.You should have a fresh hard drive ready for format and Windows use.

via Daniel Bs Tech Blog: Remove EFI Partition in Windows 7.