Disk imaging
Disk imaging is making a sector-by-sector copy of a storage device so later work is done on the copy, not on the original.
An image is a file, or a set of files, that stores the copied sectors, usually with a record of which sectors could not be read. Copying documents in a file manager is not an image. A file copy only reads what the file system can already name. An image reads the device underneath that map.
How does disk imaging work?
The copying software asks the device for each sector and writes the answer into the image. On a healthy drive that is a straight read from start to finish. On a drive with bad sectors the copy has to skip or retry the unreadable spots and record the gaps. Otherwise one bad area can stall the whole job.
Why work from an image?
A repair, a file-system check, or a rebuild writes to the disk. If that write is wrong, the original state has gone. Reading an image leaves the original device as it was, and the same image can be read again if the first attempt needs another approach.
What is the difference between an image and a clone?
The two words are often swapped in everyday speech. On this page, an image means the copy stored as a file. A clone means a sector copy written onto another drive, so the second drive can stand in for the first. Both can be sector-by-sector copies. The words describe the copy, not a particular company's process.
What should you do if a drive is already failing?
Do not run a disk check that writes to the disk, and do not repair the volume in place, until there is a copy you can afford to experiment on. If the drive is clicking or will not spin, imaging it with ordinary software can make a physical failure worse. A drive that still reads, with a deleted partition or a damaged file system, is the usual case for taking an image first.
What might you notice?
- A partition was deleted on a drive that still reads
- A file-system check is offering to write a repair
- The disk has scattered bad sectors and still responds
Related terms
Common misconceptions
Is dragging the Documents folder to another drive an image?
That copies the files the file system can still name. It does not copy sectors the file system has lost track of, and it does not record which sectors failed to read.