Before it gets forgotten: August 2016

Why do need to connect to your linkstation via a serial console?

You can see the actual messages the linkstation prints when it is booted
You can log in to the linkstation without using the network interface
Just for fun :)

What do you need?

Patience
soldering skills
a serial cable. (I have a cheap PL2303HX USB to TTL serial cable ordered from ebay)

If you are a amateur to soldering, my recommendation is do not try this.

Main board modifications

First you need to disassemble the linkstation and remove the main board. Then you have to connect four points in the main board as shown in the picture. These four holes can be found between the processor and the SATA connector.

This is where you need patience and soldering skills. It's bit difficult task to connect the holes using small wires.

Picture Source: http://home.arcor.de/gugelhupf2002/serial-Dateien/a.jpg

Serial Header

The serial header can be found near the SATA connector as shown in the picture. You can solder a pin header so that you can easily plug a serial cable.

Picture Source: http://forum.nas-hilfe.de/galerie/image.php?album_id=47&image_id=201

Once the main board and serial header are modified, you can plug a serial cable and use the screen program to view the console.

I used PL2303HX USB to TTL serial cable and didn't attach the power cable (red color) to the serial header. The white color cable connected to TxD pin, The green color cable connected to RxD pin and the black color cable connected to the GND pin

screen command I used,

screen /dev/ttyUSB0 115200

console output,

If you need to find u-boot output messages, you can do another modification to the main board. However this is extremely difficult. You shouldn't attempt to do this unless you have good experience in SMD soldering.
The modification is, remove R60 and solder it on space allocated for R59.

I almost screwed up my main board trying to moving R60 to R59. It took hours to make things worked.
You have been warned!!!