Thanks for the replies. No, I am running rockbox from the main flash memory, not gone trying the SD card (which goes in and out!). I do have a fido as backup, for when it finally fails.
However problem found/fixed - the database (which I rarely, if ever, use) was apparently corrupted, deleting all the database_xxx files solved the problem. One of them, I forget which, but it has a 3 part name, so not one of the 8 or 9, was very big, and maybe causing the issue.
I have not changed the firmware for a while, but maybe some database option got turned on/off, or maybe it just got cross trying to index yet one more audiobook.