I recently have started to experiment with Verbatim DVD+R DL with my Pioneer DVR-660H-K. I made a series of disks using high speed transfer from the HDD. I chose the video mode and finalized each of them

When I attempt to make a copy one of the disks using the recorder's Disc Backup feature. It reported that this was not supported. Then I tried using my PC and it's software and was unable to do this as well.

Can anyone help me find a way to easily make an exact copy of these disks?

I have a Pio 640, which is very similar. In my manual, it clearly states that you cannot use the "Disk Backup" function on DL discs. For this reason, I always scan through my DLs, before I delete the source from the HDD. You will need to make any additional copies you want directly from the HDD. Another alternative - after you burn a Video Mode DL disc,* split the title into two roughly equal sections. Burn each one to a SL disc in VR mode. In the future, you can dub those back to the HDD, in HS. Then, you join them to be one title again in the copy list, and you can burn a new DL disc, at high speed, in Video Mode.

Another peculiarity of DL discs - for any title that spans the two layers, you will need to pick a thumbnail from the part of the title that will be on the first layer, otherwise, you will get the last frame of the first layer as your thumbnail for that spanning title. However, any title that is totally on the second layer, you can select a thumbnail from anywhere in the title.

Why your PC won't copy it, I can't say. I have no experience with PCs, making DVDs. Will your PC copy other DL discs?

* Note: you can't make a VR mode disc from a copy list that has already been used to make a Video Mode disc. So, to do the procedure I recommend above, you will first want to initialize a disc in VR mode. Leave it in the DVDR. Then, divide the title in the title directory, and import the first half into the copy list, then burn to the VR disc. Repeat procedure for part two.

Then I tried using my PC and it's software and was unable to do this as well.

Can anyone help me find a way to easily make an exact copy of these disks?

Try using DVD Shrink. It should copy your disc to the pc, providing that the disc itself isn't bad.
http://www.afterdawn.com/software/video_software/dvd_rippers/dvd_shrink.cfm