How can I change the BIOS serial number in VirtualBox?

http://www.virtualbox.org/manual/ch09.html#changedmi

Really has detailed answer for this.

You can set the bios serial number by doing this:

VBoxManage setextradata "VM name" 
"VBoxInternal/Devices/pcbios/0/Config/DmiSystemSerial" "System Serial"

The error you were getting is caused by the serial not being set as a string value, if you have a pure number you should prepend it with string: like so:

VBoxManage setextradata "VM name" 
"VBoxInternal/Devices/pcbios/0/Config/DmiSystemSerial" "string:1234"

Note: In case your VM is configured to use EFI firmware you need to replace pcbios by efi in the keys.

Not from any external settings. But you can from the code and then build it yourself:

Source file: DevPcBios.cpp
Line: 1014 READCFGSTR("DmiSystemSerial", pszDmiSystemSerial, "0");

Change "0" to whatever value you need.

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EDIT: You can apparently use VBoxManage setextradata. I have used it in the past for other type of configurations (not bios related) and didn't test this particular setting. Give it a go:

VBoxManage setextradata *YourVMName* VBoxInternal/Devices/pcbios/0/Config/DmiSystemSerial "*yourserial*"

Otherwise... revert back to source code editing and building.

Best of luck.

If you run that command, and get the error referenced by Roger, you need to back out the command by running the command without the last parameter (VALUE).

This removes the entry causing the error.

If you are unsure of what the last offending command was, open the ~~~.vbox (XML) file in the root of the VirtualMachines folder of the specific VM and review the EXTRADATA section.

Running the command-line call to vboxmanage is the best way to modify these. Editing the xml file does not stick 100% of the time.