RAIDar can see Duo 1 but cannot access it

(a) Is the IP address shown in RAIDar for the NAS compatible with your home network address range? Is there any status indication of a failure (corrupt root or anything like that?)
(b) Do you have SSH enabled?
(c) Do you have a backup?

Stephen B thanks for you support.
(a) IP address shown in RAIDar of my RN Duo is compatible with my home network. It was doing fine before the problem happened last night. The disk indicator lights are in green & no blinking. The power indicator light is on. No blinking of any indicator lights is found. I can assess the RN Duo by browsing its address and login as admin. But the connection was last for a few minutes only. Then I cannot connect to the Duo by the browser.
(b) Sorry, I have no idear about what is SSH. Can you please tell me a bit more about this.
(c) I used this RN Duo as my backup to the data of the computers in my home network. I do not have any other backup to my RN Duo.

What can I do next ...

Try booting up again, and logging in as admin. Then immediately delete all the logs - first from the status->logs page, and then the backup logs.

It sounds like a filling OS partition.

I've tried to re-boot my RN Duo. However, the power indicator light has been blinking for more than 30 hrs since the re-boot. My RN Duo has 2x2T HD. It has about 700G data. Does the RN Duo require such a long time to re-boot? It happened once when the 2x2T HDs were firstly installed.
Should I have to shut it down again?

It should boot up much more quickly. Perhaps look at RAIDar status again?

At this point I'd probably do a factory reset, which will rebuild the NAS from scratch (and delete all your data in the process). If you want to extract your data first, you can hook up disk 1 to a windows PC (USB adapter will work) and use a freeware tool called linux reader (http://www.diskinternals.com/linux-reader/). You will of course need 700GB of space to store it on.

Thanks StephenB.
I used the linux reader to backup the data of disk 1. It took me about a week to complete because there were some files that cannot be recovered by linux reader. The backup process was being halt when there was a file not being able to be recovered. Popup box appeared and waited for my response everytime. Finally, I can recover about 450G data, slightly less than original total data size.
After that, I tried the OS reinstallation to my RN Duo. It worked fine. The file structure was reconstructed. Some of the files that cannot be recovered by linux reader become available. However, the data in the harddisk is found more than before I had the problem in the RN Duo. I had about 630G before the problem but I have nearly double the data, 1.1T, after the OS installation. Appreciated if you can advise how can I found the difference in the data and remove these files.
I'm using X-Raid in my RN Duo. Thanks again in advance.

First check the SMART stats on the drives. Frontview will show them (status->health, there's a "Smart+" button each each drive). Look for pending or reallocated sectors.

You can access the full volume if you access the duo using the NAS admin credentials. One way is to select "Map network drive" in the Windows computer pane, and specify \\nasipaddress\C. Check the box to use different credentials, specifying the admin for the username and the NAS admin password.

You can then browse through the entire volume, looking for the extra files. Start with lost+found.

Thanks again StephenB.

I've checked the reallocatred sectors of both drives. They are all zero.
I've tried to map network drive by specifying \\nasipaddtess\C and checked the box to use different credentials & the admin username and NAS admin password. But not succeed. The popup for username and password keeps on appearing. I've used the correct admin username and NAS admin password. What's wrong with this ?

Can you tell me why we have to map \\nasipaddress\C as I cannot find "C" when I browse the network drive in the Map network drive window.

I find that there is an updated firmware RAIDiator 4.1.14 available. Should I have to update the firmware of my RN Duo. The firmware of my RN Duo is RAIDiator 4.1.13

You can access \\nasipaddress\c or \\nasname\c from windows explorer directly, you don't need to map the drive letter. The reason I suggested mapping the drive letter is that approach always lets you choose the username and password before you connect.

You will only be able to see the C volume if you access the NAS with the NAS admin credentials.

Updating the firmware probably won't fix the issue at hand, but it is a good idea, since 4.1.14 has some important security fixes.

perhaps try the access again, via the command line. Go to the windows search bar, and type cmd. That will open a text command box.

Enter

net use * /delete
net use T: \\nasipaddress\c /user:admin password

The first command disconnects all existing SMB sessions (it does not actually delete anything).

The second attempts to mount the C volume of the NAS to the T drive letter of the PC.

Thanks again StephenB.
I can connect the \\nasipaddress\c to my computer by using cmd. I found nothing in the los+found folder. Anyway, I've backup all the data from the RN harddisk. Thanks again for your kind support.

I've also tried to update the firmware of RN to 4.1.14 two times this week. But not success. I had got the following error messages from the error log.

"Invalid checksum detected in the update file. Update aborted." & "Problem encountered while updating your ReadyNAS device."

Does it mean the harddisks in my RN Duo is still having problem ?