Why should I use a UPS with RAID 4 or RAID 5?

If you are running a RAID 4 or RAID 5 volume, we strongly recommend using an Uninterruptible Power Supply (UPS) with your Mac and enclosure. This is one of the most important — and most overlooked — steps you can take to protect your data.

How RAID 4 and RAID 5 Work — and Why Power Matters

RAID 4 and RAID 5 volumes protect your data by writing parity information alongside your files. This parity data is a mathematical representation of your data that allows SoftRAID to reconstruct everything on a failed drive if one disk stops working.

The key thing to understand is that every write operation to a RAID 4 or RAID 5 volume involves two things happening simultaneously: writing your data to the data drives, and writing updated parity information. Both must complete successfully for your volume to remain in a consistent, recoverable state.

If power is suddenly cut while a write is in progress — before both the data and parity have been fully written — the parity information on your volume can become inconsistent. This is sometimes called the “RAID 5 write hole.” The drives are physically fine, but the parity data no longer accurately reflects what’s on the data drives.

This matters because RAID 4 and RAID 5 are only able to recover from a disk failure if the parity data is correct. If parity becomes corrupted due to a sudden power loss, a subsequent disk failure could result in data loss rather than a successful rebuild — even though your RAID volume appeared healthy.

If you do experience a major power loss or a system kernel panic, we recommend running a Validate on your RAID 4 or RAID 5 volume afterward. Validate checks that all parity data is current and corrects any inconsistencies it finds. To validate: select the volume tile → Volume menu → Validate.

In practice, you will find that SoftRAID HDD volumes very rarely need to update any parity blocks after these events — SoftRAID has safeguards in place to keep parity data current even under adverse conditions. But validating after a power loss gives you the peace of mind that your volume is fully protected.

What a UPS Does

A UPS is a battery backup device that sits between your power outlet and your equipment. In the event of a power outage, it instantly switches to battery power, giving your Mac and enclosure time to complete any in-progress writes and shut down gracefully.

This ensures that parity data is always written completely before power is removed, keeping your RAID volume in a consistent state.

RAID 0 and RAID 1 Users

A UPS is a good idea for any storage setup, but it is especially critical for RAID 4 and RAID 5. RAID 0 has no parity to corrupt, and RAID 1 mirrors are more resilient to incomplete writes. That said, a sudden power loss can cause filesystem directory damage on any volume type, so a UPS benefits everyone.

What to Look For in a UPS

Any UPS that can power your Mac and enclosure long enough for a clean shutdown will do the job. Look for:

• Enough VA (volt-ampere) rating to cover your Mac, display, and enclosure(s)
• Automatic shutdown software support for macOS (most major brands include this)
• A model with surge protection built in

APC is a widely used and reliable brand. Their software integrates well with macOS and can trigger an automatic graceful shutdown when the battery reaches a low level.