If you want to restore the original QNAP firmware for some reason, you can follow the instructions below. Please note that this will only work if your Debian system is still working. If your Debian system is broken but the machine itself works, you can use the recovery mode to restore the QNAP firmware.
There are two steps to restore the original QNAP firmware. First, you have to put the backup of the flash partitions from the QNAP firmware you made before the installation of Debian into flash.
dd if=mtd1 > of=/dev/mtdblock1 dd if=mtd2 > of=/dev/mtdblock2
Second, you have to restart your QNAP, and then run QNAP Finder to install the QNAP software again. The second step is needed because the QNAP software is stored both in flash and on the hard drive and Debian removed the QNAP software when it formatted the disk.
After you put the QNAP firmware back in flash and installed the complete the QNAP software with QNAP Finder, your device will run the original software from QNAP again.