Someone is selling an EDA500 brand new in the box on Ebay

The EDA500 is, IMHO, a terrible choice for expanding a ReadyNAS.  At one time, I had an RN516 , and EDA500, and another eSATA box that the NAS saw as an EDA500.  That created all kinds of problems.  The asking price on eBay is also ridiculously high.  You can buy a second ReadyNAS for less these days.

Perhaps there was a time when an eSATA device made sense for a NAS.  But with large USB devices now available, I believe those are a much better solution.  A second NAS is also a better choice.  Yes, the EDA500 is RAID and most USB devices are not, but that RAID is a part of the problem because the RAID is not "self-contained" like with a RAID USB chassis, the ReadyNAS itself uses the same software RAID technique with it as with volumes within the main chassis.  A second NAS is, of course, also RAID.

An eSATA multiplier, like the EDA500 uses, multiplexes the traffic for multiple drives over a single SATA2 interface.  "Multiplier" is a clever, misleading, name -- it doesn't multiply (adding speed and throughput), it multiplexes (sharing existing speed/throughput).   When an eSATA multi-drive device is not RAID, then the traffic is generally going to/from a single drive at any given time, and it works OK.  But with RAID, it's communicating with all essentially simultaneously, and that quickly saturates the interface.  With a ReadyNAS, that's especially apparent with the abominably slow maintenance tasks which also make transfers to/from it during one of those tasks painfully slow.  Used with drives with a decent sized cache, writes are not as bad as reads.  So for backups only, it's a little less problematic.

But there is an even bigger problem, which is what happens if the eSATA comes disconnected.  The connector choice for eSATA is, IMHO, a poor engineering choice and comes disconnected too easily.  (Note, that was not Netgear's choice -- it's a standard eSATA connector.)  It should be something that's secured much better.  Mine did come loose (not even fully disconnected), and the EDA500 volume ended up unrecoverable.   If I had critical files on it, some or all of them may have been recoverable with expensive recovery software.  I don't know because mine contained only backups, so I just destroyed the volume and started over.  (A side note here:  An EDA500 does not expand your main volume, it has one of it's own.  So it's functionally like a USB drive or second NAS in many regards.)

Finally, the volume is an MDADM RAID with BTRFS on top.  So if your ReadyNAS dies, so does your ability to access the EDA500 volume except with a replacement NAS or one of the methods described in the forum for recovering a main volume from a dead ReadyNAS.  If you have two ReadyNAS with the same number of bays and both running OS6, you just move the drives from one to the other for recovery.  With a USB drive formatted as NTFS, you connect it to your PC.  If it's the EDA500 that dies, you can move the volume into the main chassis in place of the primary one for recovery, so at least there's that.

In short, there are reasons Netgear discontinued the EDA500, and I don't think poor sales was the only one (and at the original asking price, I'm sure sales were poor).  Most likely, the volume of support calls they caused was also a major factor.

Now, the EDA500 is nothing more than a 5-bay eSATA chassis with SATA multiplier.  So if you have a use for one of those not associated with a ReadyNAS, it may be a good choice, though not at the price asked by that seller.  Like other ReadyNAS products, it's well-built, not just plastic like many others.

One last note:  not all ReadyNAS support the EDA500.  Officially, nothing newer than the 516/716 does.  While I am well aware that many newer ones actually do support it, I don't know which ones do and do not.