A couple of things come to mind here.
If I’m not mistaken, standalone DVD recorders record in a slightly modified format, VR mode. This may confuse rippers. However, as Alan noted, there is no protection, and something like Nero or ImgBurn should be able to copy them just fine.
The other thing is to make sure the discs are finalized in the standalone recorder. While the standalone recorder can read its own discs okay without finalizing, the PC cannot (unless there is some special software to allow this which I am unaware of).