A RAID6 logical drive can tolerate 2 failed drives at most. If the number of failed drives is more than 2, data loss could occur on the logical volume where the logical drive belongs.

To prevent this, you must apply IDR (Intelligent Drive Recovery) technique. This includes setting up a hot spare (local, enclosure, or global), scanning media and online copies regularly, and copy/replace in-healthy drives.

For more details, refer to Infortrend IDR reference below:
Intelligent Drive Recovery (IDR): helping prevent media errors and disk failures with smart media scan