My own tests on my own KD43XD8305 showed that normal USB2 HDD's worked OK although my preferred choice is actually a USB2 Flash Drive because it is both neat and has minimal power consumption. I would NOT recommend a USB3 Flash Drive for this application. Sony officially say that 'speed is not a requirement' and my own testing bears that out.
The one I use is the Sandisk Cruzer Force. Not all Flash Drives are suitable... something to be aware of.
https://cpc.farnell.com/sandisk/sdcz71-064g-b35/usb-drive-cruzer-force-64gb/dp/MD01093
Although only 64Gb (there might be a 128Gb version available) you can fit a surprising amount on. An hour of Freeeview HD takes around 2.5Gb and around 0.7Gb if it is not HD. The firmware on the TV (I've not updated to Oreo by choice) has a glitch that says 10.8Gb is needed when you pick an hours program from the guide.
If you use or try a drive then you will find the TV formats it in a unique way and changes it to a GPT partition rather than the default MBR (master boot record). Its not a problem and is easy to revert back on a Windows PC should you want to reuse the drive in the normal way.
