Some manufacturers like to include a separate "data" partition as a backup or safe area in case the C: drive is infected or corrupted. To me an external hard drive offer more complete protection since it's on an entirely separate drive; a partition doesn't protect against hard drive failure. Besides, most customers who bring in such a laptop never seem to use the data partition.
I'm not sure what hidden files are on that data drive. The ones I've come across are usually 100% free by default. It should be safe to clear the partition and let C take over the whole disk.