The SD card should be formatted as a fat32 volume. In my experience, reformatting with a windows machine achieves that. Also, all new SDcards that I have bought are pre-initialized to fat32 and work fine in IoTaWatt.
For the benefit of those who may feel they are having similar problems, it would be preferable to report the original problem that precipitates the drastic action of reformatting the SDcard. The write activity of IoTaWatt is trivial compared to something like an Rpi, and in my experience the incidence of SDcard damage is extremely low. Most of the cases reported are, like this one, undocumented as to the original issues.
So the message is, please give a shout out with the initial problem before resorting to SDcard reformat or firmware upload or any other drastic measure.