A secure erase might fix it, see here. There are more notes on the procedure here. It will completely erase your disk (copy off anything you want to keep first!) and should reallocate the sectors. You might still have to replace the disk if it does not work.
Thanks, I have a couple of spare ex-NAS WD red 1TB drives so will probably make a copy of the Humax drive with a suitable Linux boot disc and then decide which drive to put back in the Humax. Secure erase would be an "interesting" experiment after I have copied files off.
Thanks for sugestions, very grateful for all the work that has gone into CF, will miss it when HDR fails and I have to "upgrade" to a newer humax.