ReadyNAS 102 No disks detected, despite disks being added.

The next step is to connect the disks to a Windows PC (usb adapter/dock is ok, of course SATA is also).  Then test the disks with Seatools.

Generally I recommend looking at the SMART stats, and not just depending on the pass/fail test results.  There is a write-zeros test (which is "advanced", and a bit tricky to activate).  But in this case it is worth running - it sometimes will pick up failures that the non-destructive read tests won't find.

If your disks are detected by Windows and are healthy, then your issue is with the NAS hardware.  If you are the original purchaser, then the hardware warranty is three years.  You might want to also check your warranty status  - and if you are the original purchaser and never registered your NAS, you should do that as my.netgear.com.  If you are covered by warranty, you can also upload your proof of purchase.

FWIW, I don't recommend desktop-class drives for NAS generally (or Barracuda in partcular).  Barracuda drives have a poor track record in RAID arrays, especially the 3 TB models.  Seagate might have addressed this issue in newer disk models, but I still don't recommend them.  NAS-purposed drives are a better choice for your NAS (Seagate Ironwolf or Western Digital Reds).  That said, if your drives pass the Seatools test, they should be recognized and work in your NAS.  But when they fail, I'd replace them with NAS-purposed models.