Yup, like Jim said, the sony discs may or may not be a problem. The only widely recommended DL media are the verbs, but you already have the sony's and they're working for you... Next purchase though, I'd look at the verbs.
I think you'd be better off actually choosing a write speed. Some folks run into problems with recommended...but always worked for me when I put it to the test. Personally, I write in the 8-10X range with 16X verbs and about 6X with 8X Taiyo Yuden. This works well with my system. 4X seems to work well for you based on your media, burner and write strategy, so I'd stay right there. I tend to write at 2.4X with DLs...yeah I can go faster, but my bkups are flawless and I'm not in much of a hurry.
You may want to revisit your External USB 2.0 Drive 7400 RPM (FAT32) and consider NTFS. This was recently discussed as it became an issue for a user with a similar setup...ext hdd - FAT32.
This was nicely picked up by signals... in post #18
Here's the referenced thread