well the manual doesn't got any information to replace or upgrade the hdd
amywho. i don't believe they put any software in the hdd, and very high changes you can just swap it for another and the tv will work with it.
plug the hdd to the pc, probably it is fat32 formatted, or maybe a linux partition (however some STB's got the thing encrypted and it could be the case on your tv)
if its not a encrypted partition then you can just fetch the stuff inside
i really don't recommend using win32 disk imager.. if the hard disk is damaged, you will have a hard time to make a copy of it (or will not be possible because it will be stuck trying to read)
if you manage to make the copy however, the "damaged" part will also be repliqued on the new hard drive, and the tv still "bitch" about it...
unless you are trying to save the recordings\stuff inside the hard drive, i recommend you to just put the new hard drive on the tv and see if it works that way