Trying to add video to unfinalized disc
 

With my Philips DVDR80 I was able to add video to an unfinalized disc until I explicitly finalized it. I am trying to do the same with my PC burner and Ulead MF 3, but it seems that MF will not re-edit the original disc. When I added the first video I checked the "Do not close disc" box, and MF warned me that the disc would not play in a standalone player. I burned the first video (about 4 minutes worth), then the progress bar started going backwards. It eventually finished.

I then selected "Edit Disc" from the menu. MF read the video from the disc, then I selected a second video to add from the authoring menu. When I reached the burning phase MF reported that the disc was not empty and prompted for a new one.

Should I be able to modify an unfinalized disc using my PC burner and the appropriate software? Did I miss some step in MF, or do I need to use some other program?

I doesn't work that way.
You author a DVD for good on the PC.
The "VR" method of video allows for continued additions on a recorder, and then it's also final when you "finalized" on the recorder.

So, is there no PC program to create +VR format on +R/+RW discs? If not, why not (because there is no demand, PC burners cannot create it, or this is a proprietary format)?

MF 3 will create -VR, but not +VR (at far as I can tell). The last time I tested on my standalone Pioneer player it would not play -VR, but it does play +VR.

VR is yet another format, and it would cost more royalties for sure. Most PC users make discs once and never again. It would be a wasted feature, as few would need it, but everybody would have to pay for it.

The best thing you can do it decompile the disc, and re-author again.
Try this:http://www.digitalfaq.com/edit/recor...corderedit.htm

Also, a disc must NOT be multisession to be DVD-Video spec.