This is actually a warning telling you that your hard drive is going to fail at any time. It probably marked enough bad sectors (to stop them from being used) and realized something will happen with the drive soon. You can replace it now or wait until it actually fails. The Hot Spare (if configured) will not take over until that drive actually fails.
To replace the hard drive, right click it and select Advanced / Fail. Uncheck the checkbox in the next window and type YES in the required box. Click OK to confirm.
The Hot Spare will now take over after a couple of seconds. You can monitor the progress of the rebuild by going to the Summary tab and selecting “Operations in Progress”.
If a new hard drive is handy, remove the failed drive and insert the new one after the Hot Spare takes over. It should start rebuilding the array with the new hard drive and the Hot Spare will be put back as HS after this process is completed.