No "Convert to GPT" option available

[amajamar]

(Sorry about the title and post not matching, but I solved the GPT problem/question: Operator error) But I have this other question. I am using HDM Pro 15 on a Win 7 64 bit system. I have four drives that are all MBR NTFS volumes with one singe active partition and the other half is unallocated. Why is it that if I right click on an some drives unallocated partition, I only have the option to Test the Surface, or Edit/View Sectors, but I cannot create a volume. On other drives unallocated partitions the option to create a volume is available. What would be the difference in the drives that would create the difference? .


Thanks in advance!