This particulare DVD is very sensitive to the media that you are using. It is a 2x DVD-R writer and a 1x DVD-RW re-writer. I would take a look at the firmware version that you are using on the drive (DVDInfo Pro, as well as NERO, will provide this information). If you need a firmware update, you should be able to find it here: http://forum.rpc1.org/dl_firmware.php?download_id=1603
. A firmware update is MANDATORY if you are trying to use 4x media because it either will not work correctly or the blank DVD's will not even mount. I believe that you will need the 1031 Firmware. If your current firmware is a 17xx version then you must use the newest 1731 Firmware instead. I believe that the SAGER employs the 10xx Firmware revisions.
Now for media. As I stated earlier, this DVD is real real picky about what will work, either marginally or well. I would recommmend buying a "sampler pack" of 1x-4x media from such stores as Meritline, Shop4tech, or Supermediastore. These are all on-line "stores". When comparing prices, include the shipping/free shipping as an aspect.
If you have about $130 available, I would upgrade the drive and purchase a NEC 6500A. It is less picky and a lot faster. Rated as +-R at 8x and -+RW at 4x. It also has +R Dual Layer support at 2.4x. Try Newegg and Amazon online stores for current pricing and availability. I wrote a very detailed review of what media works at what speeds for this device in the Sager General Forum a few months ago which may be of interest, should you want to go the NEC 6500A route.
Hope this helps you out some.