Welcome to the forums Optical Yankee.
The Ridata DL discs are not well regarded in these forums. We've seen many failures from double layer discs over the years, and the only ones that we recommend are the ones using Mitsubishi dyes and the 2P manufacturing process. These are most commonly found under the Verbatim brand. There are two varieties, the slower ones commonly rated at 2.4-6x using the MKM 001 mid code, and the faster ones rated 8x which use the MKM 003 mid code.
Your Ridata discs are made using the inverse stack method. Some will argue that this method is not inherently inferior, but real world results over the course of the last seven or eight years don't support that.
So it is entirely possible that these discs have slipped from barely useable to unwriteable, even though they were not burned.
My advice is to examine all of the Ridata discs that you have burned and run a TRT scan on them using Nero CD/DVD Speed. If you see a jagged drop in the reading speed graph, you'll know that they are not holding up well.
If you wish to burn more DL discs, I strongly advise using Verbatim discs. But even using the very best available, they are still too inconsistent for me. I have switched to blu ray for larger files.
And that opens a new can of worms...reliable blu ray discs. Smart-blu discs made by FTI and sold at mediamegamall are one good choice. Panasonic discs are another if you can afford them.