Oddly enough, my box did this a few times earlier in the year. Frustratingly it happened while I was away for a week, and so missed several recordings. This spurred me into action ...
I had a few reallocated sectors on my HDD, so did another fixdisk where it found a few more (now 16 in total). I also tried a System Flush while I was at it, then reconfigured everything. One point of note is that I couldn't retrieve my schedule from any recent backups properly - they would come back in the "pending changes" column after a reboot, and a further reboot would delete everything once again. All very odd, and that was leading me to believe my disk may be struggling. However, I managed to use an older backup and got everything rescheduled after which I forgot about it and got on with life...
... until another wake up in Wizard mode event. At this point I bit the bullet and installed a new HDD ( Seagate SkyHawk 2TB Surveillance Hard Drive 3.5" 5400RPM 256MB Cache, £63 inc VAT from Ebuyer.co.uk), copied all my files and CF settings across, since when everything has functioned perfectly for the past 5 weeks. Early days, perhaps, but it seems to be happy enough.
For the record, this is it's 3rd disk, and the box is 13 years old now. I've not changed any capacitors, but I have vacuumed the dust out of the case and fan when changing drives. Reading these forums has made me mindful of the flash memory's limited lifespan, and I wondered if I was hitting this issue, but so far, so good.