Resuming a ddrescue operation with an error often stems from mismatched log files or incorrect command syntax. The most common cause is using the wrong input/output order in the resume command, which causes the tool to overwrite source data instead of copying it. Ensuring the log file path matches exactly and verifying the device paths are correct typically resolves this immediate failure.
Understanding the Resume Mechanism
Theddrescue utility relies heavily on its log file to track progress. Unlike standard copy commands that simply move bytes from point A to point B, ddrescue maintains a state map. This map records which blocks have been successfully copied, which have failed, and which are pending. When you initiate a rescue, the tool reads this log file to determine where to start again. If the log file is missing, corrupted, or points to a different location than the one used in the initial run, ddrescue cannot reconstruct the previous state. Consequently, it may treat the operation as a fresh start or fail entirely because it cannot reconcile the current disk state with the recorded history.
It is crucial to understand that the log file is not just a status report; it is the primary database for the recovery process. Without it, you lose the ability to skip good sectors and focus only on bad ones. This is why preserving the log file between runs is non-negotiable for successful data recovery. If you accidentally delete or rename the log file, you force the tool to re-scan the entire source media, which can be time-consuming and increases wear on failing drives.
Common Syntax Errors
One of the most frequent mistakes users make when resuming is swapping the input and output arguments. The general syntax forddrescue is:
ddrescue [options] infile outfile logfileinfile, outfile, and logfile paths as you did in the first run. If you swap infile and outfile, the tool will attempt to read from the destination drive and write to the source drive. This is catastrophic because it overwrites the data you are trying to recover. Always double-check the order of your arguments before pressing Enter. Another subtle error involves using relative versus absolute paths. If you change directories between runs but use a relative path for the log file, ddrescue might look for the log in the wrong folder, leading to a "file not found" error or the creation of a new, empty log file that ignores previous progress.
Verifying Device Paths and Permissions
Before attempting to resume, ensure that the block devices involved have not changed identifiers. In Linux systems, device names like/dev/sdb can shift if other USB devices are connected or disconnected. Use dmesg or lsblk to verify that the target drives are still attached at the expected paths. If the source drive has been replaced or remounted, the resume command will fail because it cannot locate the original input device.
Additionally, check your permissions. ddrescue requires root privileges to read raw block devices. If you do not use sudo, the command will fail with a permission denied error. However, running it as root also means you must be extremely careful with your argument order. A single typo can lead to permanent data loss. It is advisable to create a small test script or alias that enforces the correct argument order to minimize human error during high-stress recovery situations.
Handling Corrupted Log Files
In rare cases, the log file itself may become corrupted due to sudden power loss or system crash. Ifddrescue refuses to resume and reports errors related to the log file format, you may need to inspect the file manually. The log file is a text-based file that can be opened with any text editor. Look for obvious signs of truncation or binary garbage. If the file is severely damaged, you might have to start over, but you can try to salvage some progress by creating a backup of the log file and then starting a new rescue with a new log file name. This allows ddrescue to begin fresh while keeping the old logs for reference.
For more complex system-level issues that might interfere with device accessibility, you may find related troubleshooting steps in our guide on Troubleshooting Linux Device Not Reachable During Network (Ping), as similar diagnostic principles apply to identifying hardware connectivity problems.
Best Practices for Safe Resumption
To prevent future resume errors, adopt a strict workflow. Always note the exact command line used for the initial rescue. Save this command in a text file or comment it in a script. Before every subsequent run, compare the current device paths with the original command. Never move or rename the log file unless you intend to reset the progress. If you suspect the drive is failing rapidly, consider adding the-n flag to skip bad sectors initially, allowing you to recover all accessible data quickly before focusing on the difficult areas. This strategy reduces the risk of timeout errors during long pauses caused by reading bad sectors.
Frequently Asked Questions
What happens if I delete the log file?
If you delete the log file,ddrescue will treat the next run as a completely new operation. It will scan the entire source drive from the beginning, ignoring any previously recovered data. This resets your progress to zero and may take significantly longer depending on the size of the drive.
Can I resume a rescue on a different computer?
Yes, you can transfer the log file and the image file to another machine. As long as the device paths for the source and destination drives are identical on the new machine,ddrescue will continue exactly where it left off. Ensure the OS version supports the same ddrescue version for best compatibility.
Why does ddrescue say "Invalid log file"?
This error usually indicates that the log file is corrupted or was created by a different version ofddrescue. Check if the file is readable and contains valid text data. If it is binary or truncated, it may need to be recreated, forcing a restart of the rescue process.
Is it safe to interrupt ddrescue with Ctrl+C?
Yes, sending an interrupt signal toddrescue is safe and will save the current state to the log file. The tool handles interruptions gracefully, ensuring that no data is lost during the pause. You can resume later using the same command line that was interrupted.
How do I check if my drive is still recognized?
Use thelsblk command to list all block devices and their mount states. Verify that your source and destination drives appear with their correct sizes and identifiers. If a drive is missing, check physical connections and cable integrity before attempting to resume.