Note that I'm talking about GNU ddrescue, which supersedes the original dd_rescue. See: What's the difference between ddrescue, gddrescue, and dd_rescue?.
It depends on what you choose as the target for ddrescue. From the ddrescue manual:
If you use a device or a partition as destination, any data stored there will be overwritten.
In that case you need to move the data off your drive first, or find another drive. You can, however, output to a file as well:
ddrescue /dev/hda0 /media/external/image.img This .img will really be just raw data. It will be saved alongside your existing data on the external drive, but you should make sure there's enough space to write.