Hi Tom,
Mine does same, even to this day. Just switch off and boot again. Sometimes it takes as much as 5 start-ups, but it WILL boot up eventually.
Watch the "disk" led when starting. When off or permanently on, you can switch off immediately & try again. When it flickers on/off with the data on start-up, it is responding correctly & you can leave as it will boot properly.
Cheers mate,
Henni
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Make sure you'll fly forever!