Just a quick question about RAID 0
can you run RAID 0 on a hard drive without losing its existing data?
There is no requirement for the drives to be the same size. Creating the array will use the smallest size drive as the template and waste any remaining space in the larger drive. For example creating a raid0 with a 1tb drive and a 2tb drive would yield a 2tb raid0.
you can run raid0 on a drive without losing its data. This is called a backup which should be done regardless if your running raid or not.
Always remember rule#1 - never trust 1 drive (an array counts as 1 drive) to keep your important data safe.
All drives fail so the more copies you have of your important files on different devices the safer it is.