Does it fragment or otherwise "mess up" a CF or SD card to edit and delete individual files residing on it? Will editing text files on the card mess it up? How about resizing images? On an SD card I'm currently using such activity seems to be ending up in more error messages than normal. Or is it just a defective card? Thanks!
Fragment, yes absolutely, however CF has a universal seek time (no moving parts) so it's not much of a problem.
Reads are free, writes incur a usage cost. The more writes you do on a sector, the closer to its MTBF you will get. This is tens of thousands of writes though. If the card is old you'll be closer to the MTBF. If you have got a legitimate problem then you'll find that sector is completely dead and unusuable. If sector is working still you're having other problems.