I found one of my other smaller FAT32 USB sticks (8GB). I deleted a file to free up some space and low and behold I was able to copy on to it. So, the Humax seems to be back working again and, as you suggested, the fault seems to be with my 320GB USB stick/external drive (which I had purchased from a reputable dealer)
But as I have copied files onto this USB stick/external drive previously and the Humax does not say “cannot copy files due to insufficient storage space” I remain baffled and think from now on I will just stick to using a few of the smaller 8GB sticks.
Thank you