Mine got stuck at 7%. I left it to it's own things and it eventually just told me that there was an error resetting, and from then on out i just got a continuous boot loop.
Did you find a solution to this? I bought my i5 SP4 a few days ago and basically cannot get it to work anymore. It updated to the November update and after that when booting it would log me in to a black screen with a tiny bar at the bottom and no buttons would respond. I tried to reset it, first via the built in recovery and then via a USB drive, but I am no stuck in a boot loop. The resetting takes ages and then just ends with "there was a problem resetting" and nothing else for me to do.
Q - how do I reformat the drive? I have not gotten an question about this when selecting to reset the SP4, regardless if I wanted to only reset all files or clean it up properly (the option that can take "hours").
Frustrated ... guess I will have to return it for a new one if I can't fix this.
/(Björn