Hello Mr Suter,
WD My Passport AV-TV works fine for me for recordings on all my Panasonic TVs via their USB port. These WD My Passport AV-TV hard drives were also manufactured for such recordings, among other things. However, I have no experience with Philips TVs, but this is guaranteed to work exactly the same.
In my opinion, you cannot continue to transfer your recordings on the UPC Horizon box to the aforementioned hard drive on the TV (also a question of film rights). Recordings from devices connected to the HDMI input of the TV (in your case the UPC Horizon box) do not work for me in any case. Therefore, the TV "only" allows recordings from its internal tuners (DVB-C, DVB-S/S2 and DVB-T/T2). However, when I try to record via HDMI, the message "This function is not available" always appears. I hope I have helped you a little with this.
Greetings from Bern