Raid 5 broken, NAS won't boot, how to proceed?

[Ariser]

Hi there.

I have a TS-410. Three 1 GB disks were in slot 1 to 3. 4 was empty. One day I had to reconfigure the network a lot and had some strange problems. The NAS wasn't connected to the internet for more than 100 days and during my attepts to fix the network problem the NAS insisted in updating the firmware after having established an internet connection. My fault, I clicked ok.
After mulling on the update for an extended period and some reboot attempts (which I did not overlook or even monitor closely), the disk number 3 got red and the NAS didn't come up again. I.e. it was not available via Web interface. The NAS could be pinged and the http-port was open but did not turn back a usable website but only a white page, let alone any login screen.

I removed all disks and booted without and came to the setup screen, telling me, that without disk nothing could be achieved. So I came to the conclusion, that the hardware other than the disks were ok. I then bought a disk as replacement for the red one and another one to bring the system up.
I installed the latter into bay 4 and started installation, which worked fine.
So, how do I get the former RAID in bay 1-3 up and running again?

[dolbyman]

inserting disks 1 and 2 should have brought back your NAS (in degraded mode)

Insert them, boot the NAS, if your NAS and data come back, do a backup and from then on always do regular external backups (you can schedule them automatically)

After you have a backup, insert a fresh drive into slot 3 and let the RAID rebuild