There are two major steps: installing Rockbox bootloader to M3K internal memory and installing Rockbox itself to microSD. To install bootloader in manual way follow https://forums.rockbox.org/index.php/topic,53858.0.html To install bootloader in easy way just download modified firmware update file M3K.fw and make firmware update with it in usual way. Then you need to download latest (daily) Rockbox build https://build.rockbox.org/data/rockbox-fiiom3k.zip and extract content folder .rockbox to your microSD card.

WARNING: Currently Rockbox requires FAT32 filesystem on your microSD card. You have to format your microSD card to FAT32 filesystem. Windows 10 and 11 do not allow you to format to FAT32 if your card is over 32GB. Take a look for more explanation https://answers.microsoft.com/en-us/windows/forum/all/format-a-sandisk-extreme-64gb-micro-sd-card-to/ff51be64-75b9-435f-9d39-92299b9d006e You need to use third-party utility to format your 64GB, 128GB, 256GB etc. microSD card on Windows 10 or 11.

Here is step-by-step guide:

1. Download latest native Rockbox build https://build.rockbox.org/data/rockbox-fiiom3k.zip

2. Unpack rockbox-fiiom3k.zip archive and copy the resulting .rockbox folder to the mircoSD card.

3. Copy modified M3K.fw firmware with multi bootloader to the microSD (download here https://www.mediafire.com/folder/fcc89bmjdnubs/new_bootloader )

4. Turn off the player. Insert the microSD card into the player.

5. Press the "Volume Up" and "Power On" buttons together. The update process starts.

6. Done. Now "Power On" button will launch the native Rockbox.

Notes.

• It is located only on the microSD card (!) And does not use the player's internal memory. If you use multiple cards with FiiO M3K, copy .rockbox folder to each card. So, without a microSD with a .rockbox folder, the native Rockbox will not start.

• To run the original firmware on the M3K, turn off the player (if Rockbox is running) and turn it on with the "Play/Pause" button held down (i.e. long press "Power On" and "Play/Pause" together).

• The recovery menu of the Rockbox bootloader is started from the off state of the player by the pressed "Vol+" and "Power On" buttons.

• The recovery the original firmware that usually use to perform the firmware update process with M3K.fw file will now require holding of three buttons from the off state, that are "Play/Pause, "Vol+" and "Power On".

Check the daily builds page https://www.rockbox.org/dl.cgi?bin=fiiom3k for updates (see changelogs) and update the Rockbox build manually. Just download new zip archive, extract .rockbox folder and replace the content on microSD card with new ones.

2/2/2023 Edit: updated download link to M3K.fw

2/15/2023 Edit: warning about microSD FAT32 filesystem requirement