There are a number of ways you can go. Lets cover those about continuing to use the NetApp.
The disks are normally formatted with 520 Byte sectors on storage arrays and this is different to after market disk – so they would not work.
If you want to continue in the out of warranty environment and just eke out the 840 then there should be a reasonable source of NetApp used drives available from EBAY and some refurbishers.
You could also probably get support on the F840 from NetApp, while you may consider that expensive, consider what the value is of the service neeeded – and sometimes you need to park it while you fix other stuff.
You can investigate moving the disks on to the FAS3020 and just getting support for the shelves.
The F840 probably does not have much data – the disks are 72GB and you can get 450GB FC or 1000GB SATA so why not buy new disk for the FAS3020 and migrate the data?
Or rebuild the volumes on the F840 leaving lots of spares and then just let it erode untill you run out of spares. you can follow this process and slowly reduce the work you do on the F840 and all it will cost you is electricity and cooling
I hope you are aware of the software richness of the NetApp – as replacing it simply on cost grounds may reduce your operational capability.
Most resellers will chat through your options for free if you call politely.