[RAc]

you mean an SD card? That may be your problem. SD cards made for consumer use are known to have severe quality problems which may manifest in infinite and unpredictable ways. Been there, done that. You may want to switch to industrial grade SD cards,

Thanks for your suggestion. I won't rule it out of hand but it is a decent SD card (Samsung EVO plus), has been fine for the two years that the Chromebook has been in use, and isn't used for storing MobileSheets data or any apps.

A brand name or type is no gurantee for or against anything, it may be conterfeit or mooned:  On MicroSD Problems « bunnie's blog (bunniestudios.com) . Again, I have seen examples of SD cards that looked like (and cost the same as) highly reputed ones but were trash. I do not even want to estimate how many work hours went into trying to debug seeming file system software issues that were in reality storage bugs.

At the very least you should try running a checker over the drive. May save you a lot of head scratching.