thank you Black Hole for responding and providing that link. I'll go there.
Also, thank you prpr for your questions. The hard drive is Toshiba Aug 2019 DT01ABA100V 1TB. The full picture - First the Humax booted up, then after less than a minute, black screen followed by booting again, continuously looping like that. I tried it without the aerial connected and I could see all my recordings on a different source, all recordings shown as boxes, not as a list like normal. I could play the recordings. I then followed the advice on here, opened the Humax and disconnected the hard drive and reconnected to the TV with the aerial. I could then watch the TV through the Humax but obviously not record anything. This is when I came back to this thread and put my question. Sorry I didn't give enough information. Is this enough to detect where the fault lies?