Detailed discussion and legacy firmware links for the S7-721u can often be found on community forums like 4PDA (note: use a translator as it is primarily in Russian).
or is plugged into a power source before attempting these fixes. huawei s7721u recovery image verify failed link
Place the UPDATE.APP file (extracted from the official firmware) inside that dload folder. Power off the tablet completely. Detailed discussion and legacy firmware links for the