How SD card going bad looks like a broken DAC

An object lesson in SD cards.
My Volumio install looked bad when only one channel (L) came through. Shut it down, swapped L & R cables round at HFB DAC end, powered up and startup sound came out of L channel… WTF? Has amp gone bad? Take interconnect off and ring through; nope, all good, no short-circuits. Switch to different input on amp; R channel only comes through. Hmm, insanity beckons. OK, switch OSMC RPi onto same amp input; same track comes out in L&R stereo as desired. OK, SD card bad; format, re-write and re-load. Nope, only R channel. OK, throw SD card away, format spare, rewrite and reload. Nope, starttup sound only out of R channel. Both SD cards bad? Unlikely, so start looking at replacement DACs (have hacked at HFB DAC, so slightly suspect), but just in case order some replacement SD cards (much cheaper than new DAC).

TL;DR always suspect SD card if RPi/Volumio stops making sense.