First, a wonderful tool: http://www.synology.com/support/RAID_calculator.php?lang=us
Yes, you can increase the space in your system by adding additional drives. There are a few caveats or things of note:
- Your initial choice of the redundancy configuration has an impact on what happens.
- If you have Synology Hybrid Raid, then the new drives must be at least as large as the largest current drive. (New space will be efficiently used if new drive is larger than your old drive)
- If you have Raid 5 or 6, then the new drives must be at least as large as the smallest current drive. (If your new drive is larger than your old drive, some space on your new drive will be unusable; Raid 5 or 6 will use the same amount of space on each drive as the smallest drive in the RAID)
- A temporary issue is that expanding a system may take a long time depending on how much data you have (many hours or potentially many days); the system is accessible during the expansion process but will be significantly slower.
- It is also possible to increase space by replacing smaller drives with new larger drives. You probably would not want to do this unless you have reached the max number of drives in the machine.
I recommend starting with Synology Hybrid Raid with 1 disk redundancy: http://www.synology.com/support/tutorials_show.php?lang=enu&q_id=492
That would however mean that if you have N drives, the space of N-1 drives will be available. In particular, see http://www.synology.com/support/RAID_calculator.php?lang=us
Small note: in SHR, once you choose 1 disk redundancy, you cannot change to 2 disk redundancy. (This shouldn't really matter for many people.)
Relevant information if you are adding drives to add space: http://www.synology.com/support/tutorials_show.php?lang=enu&q_id=559
Relevant information if you are replacing smaller drives with larger drives to add space: http://www.synology.com/support/tutorials_show.php?lang=enu&q_id=560
On a side note, my DS412+ died a few days ago. It would consistently abruptly lose power about 3 minutes after booting regardless of whether any hard drives are in the machine. Thankfully, the machine is still under warranty. Synology technical support was quite prompt with diagnosing the need for a RMA. I just shipped out the machine today. Let's see what happens next.