Maybe I'm wrong but I think I read somewhere that using DD could be dangerous. I think that this was the reason to switch to TAR but indeed it's a very odd problem. I'm reflashing my device right now. I don't want to lose ~20MB not even knowing what exactly is happening.

Maybe using the USB mode and DD from computer could be a solution. Wouldn't be so easy and convienient as just selecting the image from the MyDocs folder but I'm ok if is an option.