Not sure about this one. I know that HP recovery won't accept RW DVDs for obvious reasons and an USB flash drive is rewritable.To be on the safe side can you restore to an earlier time to create the 5 or so DVDs restore to factory settings discs? Once you have them, and verified, then you could play about with the Flash drive. If you have Acronis or Paragon HD Manager YOU SHOULD ALSO CREATE AN IMAGE ON AN EXTERNAL DRIVE.Have a look at the info on your PC about how to restore to factory setting(usually called restore manager or similar) using the restore partition if you have no system restore points.
Sony Xperia Z5 review: Hands-on with the phone which the Z3+ should have been