I have a question about this. In an earlier reply Computersavvy states that "Lastly, you should NEVER use dd to copy from an active (mounted) partition, and NEVER use dd to write to an active (mounted) partition. Doing so is certain to cause file system corruption either in the image or in the destination or both."

I understand unmounting the source, in this case /dev/sda7. But how do I create an image file on a target drive that is not mounted? You say:

Using the following command I create an uncompressed image:

dd if=/dev/sda7 of=sda7.img

I'm assuming that you execute this command from /home/userid, which has to be mounted.

I feel like this is a stupid question. Sorry if it is.

Saving to an image file is not the same thing as saving to an active (mounted) filesystem.

Using dd to write to an image file i.e of=sda7.img is akin to copying data to a file. Nothing out of the ordinary. You just need sudo permissions to access the device i.e /dev/sda7

Writing back to the partition /dev/sda7 i.e of=/dev/sda7 also requires sudo permissions but your writing to the device i.e partition but not the filesystem. Your actually copying the entire filesystem so if it is mounted at the same time you would be overwriting physical data but the operating system stores information in cache and if it changes unknowingly could corrupt the data being written.

That explains it. Thanks.