Its not intended that you keep old recordings, you are expected to watch then and then delete them.
If you want to keep FreeView recordings, then you need to look at using a PC with a TV card, either internally or USB. Then use a free program like NextPVR, and record and store your content on a hard drive.
There are plenty of small PC modules now, which will support a USB TV card, and have HDMI outputs which you connect into your TV.
Humax’s other recorders allow you to copy your recordings (SD only) to an external drive which you can then use to watch on another device or simply archive. I have a ton of recordings taken off a Humax (non Youview) Freeview + recorder which now reside on my NAS which I can stream to a plethora of devices. Panasonic makes a Freeview + HDD/DVD recorder which allows you to do the same thing