I would suggest a bit of caution before committing Sandisk Extreme SSDs to use as backups: I read that there's a couple of class-action lawsuits pending in the US concerning failures of these devices and according to one source, the problems are not to do with the firmware (and are hence not solved by the firmware updates that WD/Sandisk have issued) but are hardware-related. WD have apparently bought out Sandisk. Have a look at:
https://www.digitaltrends.com/comput...avoc-revealed/
PBW