Just an update, exactly 3 weeks after fixing my box the missing hard drive error reoccurred. I powered down and unplugged the box overnight but when I switched it back on the hard drive still couldn't be seen so I tried the heat gun fix again.
On this attempt I heated the heat sink chip area non-stop until the box cut out. Once the box restarted the hard drive was still not visible so I powered down and unplugged the box and gave it 10 minutes to cool down. Once restarted the hard drive and all recordings were visible again.