Why are both my machines spitting out DVD+R DL?

Ok, having a ton of problems here so I'll start from the beginning.

Using a mid 2007 iMac with leopard is preventing me from using software that requires at least snow-leopard or newer.

I got a copy of Mountain lion and discovered I cannot install it unless I have Lion, and I cant install lion unless I have snow leopard.

I now have a dmg of snow leopard on my computer. When I try to open it, it says I must first burn it to a disc.

It's a 6gig file so I purchased Verbatim lightscribe 8x DVD+r DL which have 8.5gig of space.

I put the disc in my machine and after about 30 seconds it spits it out. I then try to put it in my macbook which already has snow leopard, it also spits it out.

In my about this mac section, under disc burning, DVD-write, it says this:

DVD-Write: -R, -R DL, -RW, +R, +R DL, +RW

I tried using disc utility to burn to an external drive but right as it begins to burn it says "Could not validate source - Resource busy"

I'm really frustrated here so any help would be great.

Where did the DMG of Snow Leopard come from?

What brand and exact type are the Dual Layer DVD's?

The CD's are Verbatim lightscribe 8x DVD+r DL 8.5gb
The DMG of snow leopard is from an external drive that my friend gave to me, but I moved it on to my desktop.

Purchase a SL disc by calling Apple's 800#.

Then..

Purchase ML via the Mac App Store.

I think your iMac is having an allergic reaction to your pirating. As MyMacRox says, there's an easy (and inexpensive!) solution that is a better option.

The Dual Layer blanks I have had the best luck with in Macs are TDK's. Never had one that would not write in any of my Macs with a Dual Layer drive.

Agreed with Chas, That DMG from your friend is not going to work. If you call Apple I am told you can get 10.6 for $20-30.

Have Mountain lion successfully installed and no mac computer of the 3 in my home will read these discs. I ended up just having to eject the disk that my DMG created and was then able to burn the DMG to an external drive.

The DMG of snow leopard was purchased from a friend. I had little interest buying a new copy as once installed I was going to immediately upgrade to lion.

It's got to be something to do with the burn then. Only thing I can think of.

Do other DVD's and CD's read fine?

They do, and it's not a burn it's a blank disc being spit out

Wait, hold it, stop.

You "purchased" a DMG of Snow Leopard??

Uh, no. At least, not legally. Your friend is pirating by making DMGs of his own SL disc (or someone else's) and selling them. As a friend, you should advise him that he can get in a LOT of trouble that way.

Just buy the legal copies. Also, Lightscribe DVDs are not the same as regular DVDs, so I'd throw those out as well.