As Azbat said, I have one.
I've got a DS214 and I love the hell out of it, it's the best thing I've ever purchased and use it for far more than simple network storage. Having said that, I'm not sure about your specific question. My guess is that what you would really want to do is buy the DS216 and the additional drive (the "2" should represent 2 drive bays), and set up the DS216 with the new drive. Once you've installed the DSM software on the new drive and configure the NAS, you'll have options to add the additional drive and should be able to save all the files that exist on it.
Like I said, I'm not positive about that, but I've my 6TB of storage on my DS214 are getting close to max and I've been looking at expanding. Since my drives are currently in Raid-0, it's a difficult task. But from what I've read about my situation, the solution is to buy the expanded unit and begin with a virgin drive and then add the others with data after the initial configuration. I would imagine the process would be similar for your situation.
Two things I've learned that I'd like to share with you regarding a Synology NAS (and maybe not specific to Synology).
First - Make care to note the processor. Many "packages" for Synology are available, but they are all processor specific. You might read a guide that shows you how to do certain things only to find out that there isn't a package readily available for your specific chipset. As you are looking at a DS216, it could be that many things that DS214 owners can do, you cannot. (If you are just using this as pure network storage and aren't running any software on the NAS itself then you can ignore this.)
Second - Expansion. The DS216, like my DS214, is not expandable. You are stuck at 2 drives, and that is it. Other models, allow for expansion units via eSata. Now, this isn't exactly true because you can still expand your storage through USB but the DSM won't "see" the expansions are part of the same file source, so you'll have to be selecting different file sources for your different drives. This means if you're running something like Kodi, you'll have to manually segregate your different file types to the different devices because they won't be recognized as one cohesive unit.
It's just something to consider, I didn't really think I'd burn through 6 TB of storage, but 2 or 3 years later, yep.
I guess the thing that I want you to take away from this is that Synology is hyped so much because they have their own DSM and their own file systems, they're own software RAID implementations, etc. So that other people's experiences with NAS are not necessarily going to translate or be helpful.
My advice? Buy the DS216 (as long as you realize that expanding beyond 2 drives will be difficult) and your new second drive, set it up with the virgin drive and then plug in your existing drive. When you go into the DSM it will give you options on how to configure that new drive and it will say if you do such and such this will delete all existing files or not. However, I'd also like to add that if it does "merge" them together the process will likely take hours to complete, so make sure you have a UPS so that nothing happens during this lengthy conversion process.
I don't know if any of that helped, and honestly, if you aren't going to run any of the synology packages or take advantage of the system being more than just "Network attached storage" there are many far cheaper solutions for you to consider as well.
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