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The manual mentions a CRC check in the EEPROM, but I'm not sure if it protects the serial number or not.

If one alters the EEPROM, the serial number reported over USB is changed. The calibration and serial numbers are stored in the internal EEPROM of the dsPIC30F. It turned out that the first few words of the dsPIC's flash were set to 0x000000. Also, I was unable to connect to the DMM via USB. The symptom was a blank display and no beep when power is turned on. I've repaired a 34405A by re-flashing the dsPIC30F microcontroller. I really appreciate your help to fix my unit.

BTW, I've read somewhere that this DMM was not an Agilent development, but I wasn't expecting to see a "Jing Li" mark on PCB. I was wondering if some one here can try to read them and help me to rescue the unit!!! I'm not sure if code is protected but in any case, maybe the one on the "programmer" will be readable. I have seen it has a Microchip dsPIC and an AT91 microcontrollers. it hasn't powered on again, so now I have a 400 bucks thing with about 5kg on my desktop. I wasn't able to connect it again so I decided to turn it off and on again because the upgrade was done till "reset" so I thought that fw was already uploaded. The updater gave me an error and the DMM was in "download" mode but not responding. Everything went well until last phase when it resets the unit. After many attempts and 4 computers I finally were able to connect it and do the upgrade. Well I decided to upgrade it from the original firmware to last one. Hi guys, I'm pretty sure most of you knows that this one had a lot of problems with usb connection.