LS420 Power LED is Flashing Continuously

[pdata2]

LS420D0202
The bluish-white power light is blinking continuously. No other LEDs light up.  It started doing this after the power went out for a short time. 

When I remove power and restart, I can hear disk access for a few seconds, and then nothing.  NAS Navigator 2 cannot find the LS420. 

Unfortunately, I haven't backed up recently, so I don't want to put the data at risk.

[1000001101000]

That usually means the device was able to access the boot partition and load the boot files successfully but failed some point before activating the network or changing the power LED to solid.

It's possible the power being cut caused filesystem corruption in the rootfs. The OS may have attempted to identify and repair that corruption at start (fsck), if it has succeeded I would expect that to complete within a couple of hours.

If the data is sufficiently important you may want to consider professional data recovery since anything you try yourself involves some degree of risk.

Personally, I would connect the drives to a PC running Linux and use mdadm to assess the health of the RAID array and then attempt to start it and mount the underlying XFS filesystem and copy data directly from it. Once you have the data you need you could wipe the drives and do a fresh install of the firmware. This gives you the opportunity to assess the health of the drives.

You could also try booting from TFTP and doing a normal re-install of the firmware, that should work if firmware corruption is the only problem. If you try it do not enable debug options in LSUpdater as they will wipe the data from drives. I have some notes about the TFTP process here:
https://buffalonas.miraheze.org/wiki/Restoring_Stock_Firmware_via_TFTP