How do I fix degraded RAID 1?

What is RAID 1?

RAID 1, also known as disk mirroring, is a redundancy technology used to increase fault tolerance in storage systems. It works by writing identical copies of data to two or more disks. If one disk fails, the data remains intact and accessible on the other mirrored disks.

The main advantage of RAID 1 is data protection through redundancy. If one disk fails, the system can instantly switch to the other disk without any interruption in service. This minimizes the risk of data loss and downtime. Other benefits include improved performance for read operations since data can be accessed simultaneously from multiple disks.

A RAID 1 setup requires at least two hard disks of identical capacities. The usable storage capacity is equal to the size of one disk, as the second disk is an exact copy. For example, two 1 TB drives configured as RAID 1 will provide 1 TB of usable storage. The cost of RAID 1 is higher since twice the number of disks are required compared to a single disk.

What is a degraded RAID 1 array?

A degraded RAID 1 array refers to a mirroring setup that has lost one of its disks due to hardware failure or removal. Since RAID 1 requires two disks, the failure of one disk causes the RAID array to run in a degraded state on the remaining functional disk.

Though data is still accessible in this condition, the fault tolerance advantage is lost. Any further disk failure before replacing the faulty disk will lead to complete data loss. The array will be partially operational but at an increased risk until the defective or missing disk is replaced.

Some common scenarios that can cause a degraded RAID 1 array include:

– One of the mirrored hard disks completely fails, is disconnected, or is removed from the system. This reduces redundancy.

– There are bad sectors or read/write errors on one disk caused by aging, fatigue, or physical damage.

– One disk encounters logical/firmware errors like corruption of the partitioning or file system structures.

– One of the disk controllers malfunctions or there are issues with cabling to one drive.

– Accidental removal or disconnection of one mirrored disk from the RAID controller.

– Missing or outdated disk drivers causing one disk to be inaccessible.

How can I identify if RAID 1 is degraded?

There are several ways to identify if your RAID 1 array is running in a degraded state with one failed or missing disk:

– Your operating system, RAID management software, or the storage controller firmware will usually show the array status as degraded and specify which physical disk needs replacement.

– Many RAID controllers also have audible alarms or LED indicators that notify when a disk has failed.

– Performance of disk access operations will be slower in degraded mode as there is only one functional disk.

– Check the RAID configuration inside your system BIOS settings for status indications and details of drives connected to the controller.

– Monitoring utilities like smartctl can be used to check health statistics and errors for underlying physical disks.

– Disk management tools like Windows Disk Management can also point out a missing or failed disk based on layout and partition information.

– Online RAID monitoring software can warn about failure events and degraded arrays based on analysis of various drive parameters.

Steps to fix a degraded RAID 1 array

Here are the general steps to restore a RAID 1 array that has fallen into a degraded state:

1. Stop all I/O activity

It is recommended to stop all read/write operations to the degraded RAID 1 array before trying to fix it. This prevents any further data inconsistencies. Take backups if required.

2. Identify and replace the failed disk

Determine which specific disk has failed in the RAID 1 pair. Replace it with a new disk of the same storage capacity and interface. SSDs, SATA, SAS, FC drives are interchangeable in most cases.

3. Rebuild the RAID 1 array

Once the new replacement disk is inserted, the RAID controller will automatically start rebuilding the mirrored set. This synchronizes the data by copying all content from the healthy disk to the new disk.

The rebuild process can take several hours depending on the RAID array size. Rebuild times and progress can generally be tracked from management software.

4. Verify synchronized data

After the RAID controller completes the rebuild, verify that the status shows as normal. Check that data can be accessed properly from both mirrored drives.

The RAID 1 array will now provide full redundancy and fault tolerance again after successful rebuild.

Alternative ways to repair degraded RAID 1

Apart from the standard repair process, some other ways to deal with a degraded RAID 1 setup are:

– If the disk has partially failed, attempt repairing any software or file system errors using chkdsk, fsck, or similar utilities.

– Severely damaged disks may be recoverable using advanced data recovery methods like extracting platters in a clean room environment.

– If the disk has completely failed and a replacement is not available, data can be backed up from the remaining functional mirror disk and the array operated in non-redundant mode temporarily.

– Migrate data to a new RAID 1 array: Create a new mirrored pair with two fresh disks, copy data to it from the degraded array, then replace the old set entirely.

– Convert to a RAID 5 array: This requires available capacity to add a parity disk. Provides redundancy without mirroring.

– Upgrade to a RAID 10 array: Needs 4 disks or more, but provides better performance and fault tolerance.

Preventing degraded RAID 1 arrays

To minimize the chances of RAID 1 arrays entering a degraded state, some best practices include:

– Use enterprise-grade disks designed for 24/7 operation in RAID setups. Consumer-grade drives have higher failure rates.

– Monitor disk health statistics like reallocated sectors, temperature, vibration etc. to detect issues early.

– Ensure proper ventilation, cooling, and humidity in server rooms to avoid environmental disk damage.

– Practice regular data backups in case multiple disks fail at once. A degraded RAID 1 array provides zero redundancy.

– Schedule periodic RAID patrol read checks to detect bad sectors or read errors.

– Update disk firmware regularly for bug and reliability fixes.

– Enable disk failure prediction and alerting offered by many enterprise RAID controllers.

– Have spare disks ready for immediate replacement of failed disks to limit degraded time.


Degraded RAID 1 arrays can be easily fixed in most cases by identifying and replacing the failed hard disk. A new identical capacity drive needs to be inserted and automatic re-mirroring done by the RAID controller. Verify completion of the rebuild process through management software.

To limit the chances of RAID 1 degradation, use enterprise-class disks, follow cooling best practices, monitor disk health, perform periodic data scrubbing, and have spare disks ready for quick replacement.

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