OK, transient problem. One question is whether it may be related to a specific plot and if it might have something to do with a corruption in your datalog. I’ll take this opportunity to post some observations about this case:
This is quite an older unit. It has been running for more than three years. During that time, several problems with datalog resiliency to power failures were identified and also newer stategies were developed that very significantly reduce SDcard wear by minimizing write operations. These changes have been worked into the release stream and any unit that subscribes to auto-update has benefitted from them.
But while damage can be minimized going forward, there are a few things that can be done about wear or damage that occurred in the past.
Some SDcards have wear leveling firmware that remaps sectors with excessive write history. Consumers have little visibility into the presence or effectiveness of technology and the techniques vary by manufacturer. Older units were subjected to a lot more writes. The cards in those units were 4GB, and when the datalogs grow, there is a lot less space to remap. For the past couple of years new units have 8GB cards for just that reason.
For peace of mind, units with 4GB SDcards older than two to three years might consider upgrading to an 8GB level 10 microSD card. All you need to do is get a new card and copy the complete contents of your current card to the new. Read this post for information on removing your SDcard. Understand that there is some risk of damaging your existing card so consider that and be careful if you do so.
Most of the issues found in older SDcards were with the current log. That log contains only the most recent years’ worth of 5 second data. By now, even an older card will have overwritten any older power-fail type errors in the current log and it should be OK. The history log is another story. It will not wrap for ten years and, while much less volatile, may have errors in the early data. Should that be suspected, the history log can be deleted and it will be rebuilt for the most recent year using data from the current log.