Removing the metal backplate is simple. This is just stuck on with some flexible glue. So prise something under and off it pops.

Inside you can see a number of large cut outs in the rear of the plastic casing.

Note the small pinhole in the metal backplate that aligns with the rubber sealed hole in the plastic casing - more on that later.

There is also a very small ground out area inside the metal backplate that appears to align with the rotating mass of the vibrator.

The plastic casing is made up of two parts stuck together. Using a precision utility knife, carefully cut along the seam.

There are plastic clips at each end that are easy to cut through inadvertently if you go too deep.

The front now lifts away cleanly.

Inside the front cover you see the LCD display, attached by ribbon cable and stuck firmly onto the PCB.

Above the display, running down the right hand side of the device front, is the BLE antenna.

Lifting the LCD glass just results in destroying it, and there is nothing interesting underneath anyway.

The PCB lifts out from the back of the plastic housing easily.

Notice how the battery is supported on stand-offs from the rear of the PCB.

The battery is held in place with a small foam block adhering to the battery and PCB. Cut that away and the battery lifts off.

Now we get to see some more interesting parts.

The SoC has a QR code sticker, likely for inventory control, so take that off to see what we have underneath.

You can also see the spring-loaded paddles at the bottom of the PCB that connect to the device charging pins.

The power control module for the battery is wrapped up near the battery terminals.

Here we get a good view of what's inside:

STMicroelectronics STM32L 151 ARM-based SoC microcontroller

Nordic Semiconductor nRF8001Bluetooth chip

Accelerometer labelled 8304 AE D42 oW

Measurement specialities MS5607-02BA03 altimeter

Protection circuit module for battery

Polymer Li-Ion Battery

PCB mounted vibration motor

The air pressure hole, required for the altimeter, is protected by a piece of paper held in place by a foam collar.

In turn, the inside of the plastic cover presses onto a foam collar around the altimeter on the PCB.

This makes the device sweat proof, but not water tight.

Too bad the display was damaged during teardown.

Looks like a tidy, well-designed bit of tech!

Conclusion is that the FitBit One is not an easily repairable device.

The device is not designed to be opened, so you will do some damage by cutting it open

Taking things apart will compromise the water resistance of the device, and the LCD display is easily broken by any flexing or attempts to modify.