Now i'm a little confused. perhaps the C580 uses chapters slightly differently to my C650. Have you tried an MKV file via a USB stick or HDD ? just to make sure the C580 series TVs support Chaptering. I expect you have already. Or at least used PCSM to check Chaptering works on your TV.

All i can suggest is go ahead and try renaming the .MTA file as you have seen PCSM create it, run a maintenance and report back.
cheers

Gus