There's a page on Videohelp.com for this recorder. It indicates some issues with homemade DVDs, and suggests that copying the DVD to a DVD-RW disc can overcome the issues. There seems to be some media types that the player doesn't like, so using a different brand or speed of media may help.
It seems the errors that page shows have to do with attempting to play the discs in the player (or perhaps in another standalone DVD player). The errors received were
player says either "No title" or "Disk format error" ... if you see the same errors, you may be hitting the same issue.
If you recopy the DVD onto DVD-RW, your computer may be able to recognize the disc. If so, you can copy the files on the DVD directly onto the hard drive for further processing. DVDs use a UDF filesystem and a particular naming scheme for the files, and commercial DVDs add encryption to the data files. I don't know if your recorder will add encryption to your homemade discs (I doubt it) -- if it doesn't, just copy the DVD contents into a new folder for an instant hard-drive backup of the DVD.