A7III rebuilding database file????

[Ski542002]

Hello:

Weird issue with my 3 year old A7III. Shooting raw, card to card backup for redundancy.   End of day.  Turned A7III back on to check image count and on-screen it said something (can't remember exactly).  Rebuilding image database. Hit the replay button and nothing on either card.  I should've written the message down when it happened but I was so flustered and terrified that I lost 400 images (on both cards) I didn't. Was hoping that the data was still on the cards even though the camera said nothing there.  Sue enough.  Got home, used card reader and both cards had good data.    Any idea what could cause this?  After backing up data on my Mac, I reformatted cards, ran a huge-burst test and of course now everything functions normally.