The original 3TB drive died unexpected without any warning.

Opening was a bit challenging but with the right tools (from ifixit 😉) manageable.

I bought a new WD Red Plus 4TB on Amazon. The red Series has better specs regards temperature, on-off cycles.

The rubber needed some minor cutting to fit the new drive.

Tips: be careful with the connector cables, avoid sharp tools when disassembling and assembling.

A test before closing the lid is highly recommended.

AirPort App recognized the new drive, formatting was the only task required. The quick format is sufficient and take only seconds.

Done.