Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: still not clear - does this unit includes 2 hard drives of 2TB each?
Document: Upgraded from an Original ReadyNAS (Still Running). I put in the WD Red 3TB drives and loaded it up. Installation and directions are clear.The unit took some time readying the disks which is normal and expected.Loaded up the unit with data.
Then I remove 1 of the 4TB drives and replace it with one of the 8TB drives. After the NAS copies over all the data from the 4TB that's still installed, I can replace the other 4TB with the second 8TB and then the data from the first 8TB will copy to the second 8TB. Then I'll have increased my space from 4TB's total to 8TB's total ( that's (2) 8TB hard drives mirrored in RAID 1) of redundant space.
THIS IS SYNOLOGY'S TIME BACKUP FOR LOCAL NAS BACKUPS) service is going strong. I use my old 1TB external USB 2.0 hard drive for it currently (Eventually I will be buying a 4TB external USB 3.0 hard drive to plug in the back of the unit for dedicated Time Backups) so I have to plug it into the front USB port on the NAS.
8 months later and still VERY happy with my purchase. Synology keeps putting out great software updates too! This has become more than just a media server. Provides robust home office capabilities with the apps that Synology provides. Highly recommend this NAS. Went with the Seagate HDs and they seem to always be performing well. I did go ahead and buy a APC backup power supply. You may want to consider adding this cost because you don't want to lose your NAS or hard drives from power failure.
Most would be like WTF, I don't get 8TB??? but with this solution, if 1 of the drives fails (WHICH I'VE SEEN MANY TIMES), I still have another drive and hopefully enough time to buy a new one and swap out the bad drive. There are Youtube videos out there showing the process to replace a failed drive. The Synology makes it easy and automatically populates the failed drive for you. I'm guessing that it would take about 2 days to populate a 4TB drive.
**Updated 3/19/2013******* UPDATED 8JUN2013 BELOW *****I just received my Synology DS213+ earlier this week. 2 days ago, I received (2) Seagate 4TB 5900RPM drives to put into it. I have some pics sitting on my camera that I'll upload later. I actually received the NAS a couple days before the drives came in. It's really hard to put an accurate Star Rating on this because of my background...
I paired the DS213+ with a DX213 Expansion Unit populated with WD Red 2 TB drives.Drive installation was a breeze, just follow the well written instructions in the quick installation guide.No issues installing the latest DSM (4.1) and no setup/configuration issues.After about a week I decided to upgrade the DSM to 4.2.
Very nice.Don't use it more than a network file share for backups and file storage, so I don't use a lot of the on-board applications and services and can't speak too much about that.But, being diskless, you not only have to drop a bundle on the unit, but buy 3 drives (2 prime and 1 socked away backup...if you're doing disk mirroring) and an external USB 3.0 drive for system backup.
I've installed 2 of these in the last month. Both are being used to replace aging/dead servers. One server was hit by lighting. Since these offices have  moved to Office 365 hosted Exchange there was no need to buy a full server. It's quite, sturdy, fast and really easy to configure. Both have been up for a month with no problems.Update 8/28/13:Just purchased a third unit. It's configuring the drives as I write this.
I can say enough how awesome it is. My favorite is surveillance station for my home security cameras. The cloud share is great too, my kids have their home work on it. I have all my movies, apps and music all stored in it. I am using WD 2TB Red Drives with no issues.
Answer: It is confusing/missleading on Amazon. The DS213+ priced at $399 does NOT include disk drives.  Amazon have a seperate listing for a DS213+ 2200 with 2 * 2TB drive priced at $799 (showing out of stock on 12/15/12). I am still very pleased with my DS213+ which I fitted with 2 * 4TB drives. It certainly wasn't the cheapest NAS but so far has worked flawlessly.