How to Prevent or Repair Hard Drive “Data Rot”

When you have a hard disk connected to your computer, all is good. It periodically checks to be sure all data is secure.

However, when you unplug a hard drive (also called HDD) and store it on a shelf, the magnetized bits that store your data can, over time, lose their charge and its associated data. This process is called “data rot.”

The general rule is that HDDs containing data can be safely stored on a shelf for 3-5 years before problems of data loss or sticky heads (“stiction”) arise. So to prevent problems we need to “reenergize” the data on the disk.

Here’s how.

IMPORTANT NOTE: This requires using Terminal, because I don’t know of an existing utility that does this. This isn’t hard, I’ve run it myself. But, if you are not comfortable using Terminal, get a tech-savvy friend or consultant to help.

• Open Disk Utility
• Mount the drive so it appears on the desktop.

NOTE: If the drive does not have an enclosure, use a drive dock (like this Voyager from OWC) to connect it to your computer.

• In Disk Utility, watch for the drive to appear, then select the top layer, the one above the name of the drive (indicated by the red arrow), in the list on the left.

• On the right panel, note the drive name next to Device (lower red arrow). In my example, it is called disk11.
• From Applications > Utilities, open Terminal.
• At the prompt, enter the line below, inserting the name of the Device in place of “—“

sudo dd if=/dev/r— of=/dev/null bs=2048k status=progress

NOTE: In my case, I used: sudo dd if=/dev/rdisk11 of=/dev/null bs=2048k status=progress. That “r” before the name of the drive is important.

Terminal will ask for your password, then display a progress report on the process (red arrow above).

This command reads every sector on the disk – even if the sector is empty and even if it’s not a Mac format disk. Writing is not part of the pass since the drive firmware logic will handle sector reassignments if there are low level questionable reads. The purpose for this is twofold: keep the heads cleared of any parked “stiction” opportunities and validate the sectors on the disk via drive’s the built in block management logic.

This does not erase anything, nor write new data.

NOTE: The “sudo dd” option forces the drive to hit all of the drive’s sectors, not just those specific to any one filesystem/partition.

Most hard disks transfer data between 50 – 250 MB/second. The duration of this re-energizing process depends upon the speed of the drive and the amount of storage to be reviewed. This table can help you calculate the duration.

Time to Process 1 TB of Storage

From the table above, multiply the hours per terabyte times the total capacity of your hard drive to get an estimate of how long this will take. If your computer goes to sleep, it will take longer.

EXTRA CREDIT

I’m grateful for the technical assistance of Tim Jones, Chief Solutions Architect at OWC, in providing and verifying the Terminal instructions.