Disk 2 fail. I change it but nothing happens!!!

Hi, thanks for the help.
I make a reset to my ReadyNas and now i am able to login via user name: admin and pass:netgear1.
Now i can see the following:
At Volumes =>Volume Settings, i have RAID Configuration: X-RAID(Expandable RAID), 4 disks.
Status: Not redundant. A disk failure will render this volume dead.
(THE RESYNC VOLUME BUTTON IS GREY AND I CAN'T PRESS IT)!!!

Also:
RAID Disks:
CH1: Seagate ST32000542AS (1862GB) (Green light on)
CH2: WDC WD20EARX-008FB0(1862 GB) (Grey light)
CH3: Seagate ST32000542AS (1862GB) (Green light on)
CH4: Seagate ST32000542AS (1862GB) (Green light on)
Also, i removed and pushed in again the disk2 and at Status=>Health, i have "The disk attached to channel2 could not be used. The most common reason are RAID resync in progress, faulty drives and disks that are too small to be added to the array."
What i have to do now?
Please i am in serious problem, bacause i can see now that every day i have growing errors at disk 3.... Now i am at 668 errors.
Olease reply as soon as possible.
Thanks