I have a 27" iMac retina display with 32gb memory running Mac OS 10.12.3. I have my Lightroom photos stored on an external thunderbolt Promise Pegasus 10Tb drive. I am backing that drive up wirelessly to an ioSafe NAS drive. I have been having multiple Out of Memory crashes while the backup is running in the background. Lightroom is open but not necessarily even being actively used. I have memory apps that point out the application culprit and it is definitely Lightroom - using 20+ Gb of memory and climbing before the crash. I have been using Lightroom since version 1 and have never ever had memory crashes before now.