How do i image a partition into a file in linux?

I have a hard drive with a partition i want to save to an image so i can create a loop back device later and view the files.

I wont have access to the hard drive with the partition i want to backup later. Hence my desire.

What should i use? I tried
PHP Code:
dd if=/dev/hdb1 of=./mydrive.iso 
But it grew very large. Is this correct? I thought the iso should be roughly the same size as when you do a `df`.

 

 

 

 

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