Recovering data from crashed ReadyNAS Duo

The drives are formatted for Linux, not Windows.  It's not encryption or anything proprietary, and it is unlikely your data was wiped.  It would be a good idea, though, to test the drives with the manufacturer's tool for that using the PC connection.  Just don't do any write tests.

Does the unit show up in RAIDar?  Try it both with and without drives.

Assuming the drives were in RAID1 configuration, the NAS might come up with just one or the other installed, too.

If your 4-bay NAS is of the same generation and archetecture as the Duo, then you can use it to access the data, assuming the volume itself is not the problem.  For an original Duo, that would be an NV or NV+ (V1).  For a Duo V2, that would be an NV+ V2.  Note that the "V2" means one that says that on the front and has a -200 part number.  Duo and NV+ (v1) models may say something different on the part number sticker, and they are -100 models.  It is best to use a spare drive in the "new" NAS first and bring the OS up to the same revision as your Duo was using before you install the drives from the Duo.

R-studio for Windows running on the PC is another option for recovery, though I've never used it.

Lastly, Netgear can (for a fee) help you recover your data using a current OS6 NAS.  But you'll need a place to offload the data, they can't just convert the existing volume to operate in the newer NAS.