in the process to add the drives, it mentions to format entire disk, and doesn't formatting usually wipe your data. I'm trying to retain my videos/files without wiping

as per the subject, I'm looking to switch from Nas4Free, I have 2 3TB drives on my current nas but only using 1.7TB. Any suggestions what to do with my data instead of just buying a separate 2TB external hard drive and move the data back & forth? Thanks in advance