A 1tb is the maximum that the 4k box can use so may be falling foul as it's registering a greater than 1tb capacity
I normally recommend a 256gb for ease of formatting and an optimum size for recording, unless you like to keep series for long periods of time. Bear in mind the old YouView boxes were 350gb.
Also problems can occur if the usb sticks are purchased from a well known auction site. There are many fakes out there, and also what may appear to be genuine 1tb sticks that in reality have a much lower capacity. Because the stick may register as 1b, when formatting they will fail as the actual capacity does not match.
I'm not saying you have a dodgy stick just outlining some known issues. The 4k box has a reputation for sniffing out issues and was a feature with myself and fellow testers of this box.
hope this helps you investigate the issues you are experiencing.
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