Unless they specifically said a new HD would fix it - a new HD would normally fix nothing. If a new HD would "fix" a clean install of the OS should also fix your problem.

Often reapplying the update "fixes" the problem.

Sometimes the EFI gets corrupted ... in many instances there's a way around that too.

"It won't work" doesn't give us a lot to work with in helping you. THe more information we have the more "useful" we can be. There are three different versions of Apple TV - you didn't even specify which you have.

As for purchasing a new HD. You can get one from here or use Google Shopping.

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