How to merge USB HD partitions without data loss?

Question

I have a WD My Passport 0748 USB3.0 HD. It's partitioned into two NTFS partitions 31.4GB, 900GB. I want to merge these partitions without a chance of data loss, I need it VERY SAFELY. When i tried my traditional solution "Windows Disk Management" under Windows 8, it gave me this message:

The operation you selected will convert the selected basic disk(s) to dynamic disk(s). If you convert the disk(s) to dynamic, you will not be able to start installed operating systems from any volume on the disk(s) (except the current boot volume).

Actually, I don't know what it means, but i need to merge it VERY SAFELY.

Answer 1

Windows Disk Management cannot extend a disk if it has a lower offset than the selected partition, i.e. it is on the left of selected disk. So what to do, use the disk managers discussed here:

1. How can I extend a volume where the unallocated space is located to the LEFT of the Primary Partition in Disk Management
2. Windows - Adding Unallocated Drive Space "From the left"

Basically first the partition will be moved left then extended. See the steps in the second link.