One of the possible problems with deleting things using the computer is that computers usually buffer access to storage devices - that is a portion of the memory card is copied to the computer memory, and for a while the computer works on this rather than on the card as its faster to access computer RAM than the SD card. Eventually the 'buffer' in the computer will be written back to the SD card - but if you've deleted something, it could be that for a little while the tables used to say where things are on the card don't exactly match what the computer thinks they should. If you properly eject the card (e.g. using the Windows eject function) you'll get a message to say either the card is in use, or it's safe to eject. If it's safe to eject it means all the buffers are written back to the card. If you pull the card out before it's safe to eject, or your computer freezes or you lose power it is possible that the information on the card is a bit scrambled. Another problem that can occur (not sure about the Pentax, but it happens with the cameras I have), if you edit an image on the card (even if you just rotate it and rotate it back again - very easy to do if you plug the card into the computer and you're reviewing pictures on screen), it is possible the camera won't see the image again - cameras usually 'sign' the images (the EXIF data contains information about the camera and how it is set), and if the image changes in any way from what it is expecting (i.e. how it was when it was originally shot), it may complain.