Error Invalid FID

First time I have had any unusual behavior with my MK1 after three years with it… the sample in question was from direct sampling and saved in the general Audio folder.
I was changing in and out samples in my unused slots and got the ERROR pop up in the slot. The gain was way down and the BPM blank when I looked at the Attributes, which I corrected and re-saved the file after rebooting the OT.
I only have 3 or 4 MB left in my slot list. I assigned it succesfully to a Flex and behaves as expected. I am not completely certain what the Error code means, or what if anything I can do about it? I have other projects where I use up all but a few MB of RAM in the slot list and never experienced any difficulty.