Hi Portfire,
Welcome to the forum.
You need a hard drive that is designed for Video pvr use as per link below. (Ignore ref to Youview) There maybe other alternatives, but these are the spec to look for. You can obtain NEW & used from various online sellers, some with a warranty.
https://ionicindustries.com/repair-guides/bt-youview-box-hard-drive-replacement/
If you have a 2.5 hard drive you can easily adapt this to fit with tray & adaptor lead, BUT although drive will work, is it to correct spec? If not, it will fail faster than the recommended drives. Pvr hard drives are put under more strain as they remain constant as they use a continuous buffer for pause/life tv. Hope this helps. John L