Options for expansion RN104

While the Netgear EDA500 expansion has been discontinued, I have found other eSATA expanders that work.  They must only use eSATA port expansion, no USB, FireWire, Thunderbolt, internal RAID, etc.  Frankly, though, they are slow (because of the use of port expansion) and would be even moreso on a slow 100-series NAS.  I had two on an RN512 and have retired them in favor of rack-mount NAS with more bays.

You could use a USB3 RAID box, but I have seen none that have a Linux/NAS user interface, so you'd have to move from a PC or Mac to the NAS and it won't be as well integrated into the system.

You can replace as few as two drives and get more space.  Swapping two 4TB for two 8TB (one at a time, with re-sync in between) will give you 4TB more, and then drives 3 and 4 will each give you another 4TB when swapped for 8TB.  Spreading them out over time is usually the best plan.  It makes all the drives not be the same age (and, thus, have less chance of nearly simultaneous failure), it spreads out the expense over time, and the price of the drives will likely drop.  Save the 4TB's you remove, as they will make suitable replacments for the 4TB's that remain, should one fail (though you cannot put them back in place of one of the new 8TB).  You could. of course, go with 12TB instead or 8TB, gaining you 6TB with the first pair and 6TB each on the next two.  If you replace only a couple, take a look at the SMART stats to see if any are in worse shape than the others, and replace those.  The order of replacement doesn't matter.  Oh, and make a backup of your data before starting expansion.  RAID sync is drive intensive, and drive failure is more likely during it.  A failure during sync will almost always kill your volume (unless it's RAID6)..

Then, there is the option of another NAS.  Even the current entry level ReadyNAS is much faster than your 104.  You could use them together, or relegate the 104 to backup duty.  If you do go with a new NAS, it's still best to start with fewer, larger drives, and maybe more bays.  Then, you can expand by just adding drives, not replacing.