ReadyNas 6.2 (NAS locks up entirely)

My drives are only 10-15% full. I use them for digital photo storage and I have only noticed it happening when I've been post-processing images in Adobe Lightroom, so in reality I will have read the image from the drive, worked on it for 5-10 mins and then tried to save a new copy back, and this is when Lightroom hangs due to the drive having gone offline. When I press the power button to the display remains blank and the drive unresponsive. The only way to overcome this is a 'hard' power down by removing the power cable which is not good. I'm trying to think when it started but can't put an exact date on it other than probably within the last 2 weeks and I keep the firmware reasonably up to date. In the end I've carried out a full re-install of 6.2.2 yesterday in case it was a glitch in the update process at some point and will monitor but it's intermittent and unpredictable. Disk spin down is disabled.