I'm noticing the same behavior on my Windows 7 machine as well. This appears to have just started doing this on the 15th. At least, that's when I started noticing missing recordings. I haven't found anything in the App Event logs as well as any WMC logs. I haven't made the switch to EPG123, so this possibly rules out the idea that it's a guide problem?
My recordings go to 3TB drive on D:\. Also, I'm not performing any type of conversions of recordings. Seems really strange. Any ideas where to look?