Yeah， afterI drag-and-drop the firmware file onto the mounted drive, uninstall and reinstall the mbed serial driver, it still fail. All the other drivers are installed successfully except mbed serial driver, so I can't use the serial port of the board.

The setupapi.dev. log:

ndv: Searching Windows Update for drivers... 16:18:31.435 ndv: Acquired WU search serialization mutex. 16:18:31.436 ndv: About to release WU search serialization mutex. 16:20:59.725 ndv: No driver found on Windows Update. 16:20:59.725 ndv: Searching Driver Store and Device Path... dvi: {Build Driver List} 16:20:59.726 dvi: Searching for hardware ID(s): dvi: usb\vid_0d28&pid_0204&mi_01 dvi: Searching for compatible ID(s): dvi: usb\class_02&subclass_02&prot_01 dvi: usb\class_02&subclass_02 dvi: usb\class_02 cpy: Policy is set to make all digital signatures equal. dvi: Enumerating INFs from path list 'C:\windows\inf' inf: Searched 0 potential matches in published INF directory inf: Searched 35 INFs in directory: 'C:\windows\inf' dvi: {Build Driver List - exit(0x00000000)} 16:21:00.285 ndv: Selecting best match from Driver Store (including Device Path)... dvi: {DIF_SELECTBESTCOMPATDRV} 16:21:00.286 dvi: No class installer for 'mbed Serial Port' dvi: Default installer: Enter 16:21:00.288 dvi: {Select Best Driver} ! dvi: Selecting driver failed(0xe0000228) dvi: {Select Best Driver - exit(0xe0000228)} ! dvi: Default installer: failed! ! dvi: Error 0xe0000228: There are no compatible drivers for this device. dvi: {DIF_SELECTBESTCOMPATDRV - exit(0xe0000228)} 16:21:00.290 ndv: {Core Device Install} 16:21:00.291 ! ndv: Installing NULL driver! dvi: Set selected driver complete. dvi: {DIF_ALLOW_INSTALL} 16:21:00.293 dvi: No class installer for 'mbed Serial Port' dvi: Default installer: Enter 16:21:00.294 dvi: Default installer: Exit dvi: {DIF_ALLOW_INSTALL - exit(0xe000020e)} 16:21:00.295 dvi: {DIF_INSTALLDEVICE} 16:21:00.295 dvi: No class installer for 'mbed Serial Port' dvi: Default installer: Enter 16:21:00.297 ! dvi: Installing NULL driver! dvi: Writing common driver property settings. dvi: {Restarting Devices} 16:21:00.400 dvi: Restart: USB\VID_0D28&PID_0204&MI_01\0240000047784E45003A9004D7450001DDB1000097969900 dvi: Restart complete. dvi: {Restarting Devices exit} 16:21:00.423 dvi: Default installer: Exit dvi: {DIF_INSTALLDEVICE - exit(0x00000000)} 16:21:00.424 ndv: Device install status=0xe0000203 ndv: Performing device install final cleanup... ! ndv: Queueing up error report since device installation failed... ndv: {Core Device Install - exit(0xe0000203)} 16:21:00.426 ump: Server install process exited with code 0xe0000203 16:21:00.511 <<< Section end 2019/04/22 16:21:00.515 <<< [Exit status: FAILURE(0xe0000203)]

What's your host OS? Is it Windows 10 or macOS or Linux?

And check if this article helps,

https://support.microbit.org/support/solutions/articles/19000089574-beta-testing-mbed-driver-doesn-t-work-with-windows

I uninstall all the drivers related to the board and then reinstall, the mbed serial driver is OK now. Thank you!