I just recently bought some Memorex DVD+ DL and my new Mac Mini could simply not burn them. Turned 4 into coasters before I gave up.
I was about to think there was something faulty with the drive in my new Mini but decided to buy a pack of Sony DVD+ DL's ... guess what - no problems!
3 perfect burns later, I know one thing - I'll never buy another pack of Memorex DVD's.
Buy Taiyo Yuden- they are extremely reliable- the "That's" brand is their own production.
Never had a single problem with Taiyo Yuden disk.
From the DL disks-the Ricoh and Verbatim are best, IMHO.