Hi Yin Yang,
1. USB-HID
Did you use the flashloader_loader? Take care, this file is the once used bootloader.
After you use the UART bootloader to download the app, then you want to use the USB-HID, you need to download teh flashloader_loader to your FRDM-K66 board again.
So, please download it to your board again.
Now, you need to two USB cables, one is connected to J26, another is connected to J22.
Press the on board reset button, then click connect in the kinetisflashtool

You can find the device is connected to device successfully.
Now click update, take care, choose the led_demo_freedom_0000.bin file

The picture means the app is successfully downloaded to your board, and you will find it works.
2 USB MSC
Yes, the kboot2.0 can support the MSC bootloader, you can drag the app .sb files to the MSC device.
More details, please read one of our post at first:
USB MSD Bootloader Implementation
The above post is using the freedom-flashloader, if you still want to use the once used flashloader_loader, the same way, download the flashloader_loader to your board at first.
Plug to USB cables to your board, you will find you can find a device named as FSL Loader, this is the USB MSC device.

take led_demo_freedom_0000.bin as an example.
(1) modify the .bd file, you can write it like this:

FRDM-K66_0000.bd is used for the led_demo_freedom_0000.bin file.
FRDM-K66_a000.bd is used for the led_demo_freedom_a000.bin file, which is relocate to 0xa000.

I will attached all the according files to you, you can put it to your NXP_Kinetis_Bootloader_2_0_0\bin\Tools\elftosb\win, just for generate the according SB files.
(2) generate SB files
Still take led_demo_freedom_0000.bin, FRDM-K66_0000.bd as an example.
open elftosb, and input :elftosb –V –c FRDM-K64F_0000.bd –o led_0000.sb

You will get led_0000.sb, then send led_0000.sb to your "FSL Loader" device.
Now you will find the led is blinking on your board.
Please try it on your side.
I also attached my generated .sb files.
image.sb is used for led_demo_freedom_a000.bin.
led_0000.sb is used for led_demo_freedom_0000.bin files.
Have a great day,
Kerry
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