From NAS-Central Buffalo - The Linkstation Wiki
Power LED (green)
||At bootup, prior to ppc_uartd starting, Just prior to ppc_uartd starting to write to flash memory
If ppc_uartd is not started for some reason, it remains blinking, and then after few minutes, DIAG LED blinks 6 times(hard disk error), 6 times, 6 times, ... finally the box shutsdown.
DISK FULL LED (red)
||Writing to flash memory
DIAG LED (red)
|3 times blink
||Partition warning: /dev/hda3 is not mounted on /mnt. You can ignore it.
|4 times blink
||Problem with the cooling fan - probably rotor jammed.
|5 times blink
||Problem with the flash memory*
|6 times blink
||Hard drive not found. Or ppc_uartd is not running.
|7 times blink
||Problem with the RAM/LAN/HDD controller.
|blink many times
||Writing to flash memory.
* Sometimes, you can recover from a flash memory error by pulling out power cord and try again after one night.
Table copied from http://www.yamasita.jp/linkstation.en/led.html with minor rewording. nobuakiy helped with the translation.
Users have reported problems when the drive already has a Linux partition on it - the diag light blinks 6 times. The solution appears to be to simply to reformat the drive.