From NAS-Central Buffalo - The Linkstation Wiki
Power LED (green)
|blink||Bootup, loading Flash ROM and starting ppc_uartd.|
|blink slowly||Shutdown timer has been activated. Box will shutdown soon.|
If ppc_uartd does not start for some reason (or is started late due to hardware/software issues), the Power LED will continue blinking. After five minutes the DIAG LED will continuously blink 6 times (hard drive failure) and then shut the box down.
|blue||1000BASE-T (Kurobox HG only)|
DISK FULL LED (red)
|on||Hard drive full|
|blink||Writing to Flash ROM|
DIAG LED (red)
|3 times blink||Partition warning: /dev/hda3 is not mounted on /mnt. You can ignore this.|
|4 times blink||Problem with the cooling fan - probably rotor jammed.|
|5 times blink||Problem with the Flash ROM or Boot Kernel*1 *2|
|6 times blink||Hard drive not found, or ppc_uartd is not running.*3|
|7 times blink||Problem with the RAM, Ethernet Controller, or IDE Host Controller.|
|multiple blinks||Writing to Flash ROM.|
Source:  with translation help from nobuakiy.
*1 If your boot kernel is located on your hard drive, and your hard drive fails, you will get the Flash ROM error instead of the hard drive error because the boot kernel could not be loaded.
*2 It may be possible to recover from a Flash ROM error by pulling out the power cord and trying to boot again the next day.
*3 Users have reported getting this error when the hard drive already has a Linux partition on it. The solution appears to be to simply reformat the hard drive.This might also happen if the PSU does not provide enough power for the hard drive, or if the hard drive takes too much time spinning up.