I've been using the Thunderbay 4's for a few years with SoftRAID and have been very happy with them.  I have 2 of them set up in a RAID 1+0 so that even if 1 thunderbolt cable gets pulled by accident, or one enclosure fails the RAID continues working.

Unfortunately I wasn't expecting one of the units to fail after only 13 months (12 month warranty on the empty enclosure).  I contact OWC saying that I had identified it to be the power supply that had failed.  They told me they didn't sell spare parts, and they could not repair it. I asked if they could tell me where to get a replacement power supply from and they would not.

Stupidly I ended up buying a new Thunderbay 4, but rather than disposing of the old one I packed it away for spare parts.

Earlier this year I decided to get out my old Apple II's and 68K Mac's - all of which used to work fine the last time I used them, but may of them no longer work now.  I found online information and learned how to repair them - mainly replacing capacitors - and now months later I have everything working.  It's very satisfying - and fun to to play with those old computers.

This led me to have another look at the Thunderbay 4. Unfortunately I couldn't see a problem with any of the components on the power board - and unlike the old Apple's I assumed it was less likely to be a failed capacitor (given that they tend to fail through age I assume).  However, this time I did noticed one of the components had a label on it that I must have somehow missed last time.  It had a product code PPS-200-12.  I googled it, and it turns out that lots of places like Mouser sell them.  Whilst many places were on backorder, I even found an online shop in the UK that had one in stock.

So if anyone has an out of warranty Thunderbay 4 that the power supply fails on - the power supply board is an off-the-shelf part.

It feels like because of MacSales poor after sales support and short warranty period (a 12 month warranty isn't allowed on electronics here in the UK - as failure after only 13 months deems it as not fit for purpose) I spent around £500 on a replacement Thunderbay 4 unnecessarily.

The only consolation is that once I receive the replacement power supply I'll have a spare enclosure to be able to swap over immediately if/when either of the other 2 fail.