SMART Says BAD While a Hard Drive Slows Down: Should You Keep Copying?
If an HDD is freezing the computer, slowing down, and changing from SMART CAUTION to BAD, stop treating it like a normal file-copy task. A reallocated-sector display changing number is not evidence that the physical problem has healed.
In this r/datarecovery SMART-warning case, the owner could not finish copying files and asked whether a partly filled 4TB SSD could hold an image of the failing drive.
Stop the Workload That Is Making the Disk Worse
Cancel repeated Explorer copy attempts and avoid repair, benchmarks, long surface tests, and full recovery scans on the original. The more a deteriorating disk struggles, the more important it becomes to preserve available reads. If the files cannot be replaced or the disk disconnects repeatedly, a professional recovery service is safer than repeated DIY passes.
Size the Image by Source Capacity, Not Files Remaining
A raw disk image generally represents the entire source device, including unused space. About 300GB of files remaining does not prove that a 300GB image is sufficient. Confirm the full source capacity, destination free capacity, and the intended image format before any command is run.
If the risk is acceptable and the hardware still reads, a knowledgeable operator may use a resumable imaging workflow with a saved mapfile. GNU ddrescue explains its read-error handling and mapfile. Its output must be a dedicated destination file or device identified unambiguously; do not overwrite the existing content of the partly used SSD.