Not sure why the PC won’t see the blank disc while Toast is running, perhaps some strange thing with Toast (not a Mac user myself, so that’s just a guess!), but as long as the disc is recognised before you open up Toast, then I’d say it’s annoying but not a huge worry.
I believe the highest speed your drive can burn at would be 8x - with decent media, I’d say go ahead and burn at that speed, but with those TDK’s…you can try 4x and see if that helps - but any lower than that I don’t think would help.
The TDK DVD-R (16x) that I’ve used have either been made by Moser Baer India Limited or CMC Magnetics. Under the TDK brand, I’d say both are alright but nothing special. For best results, I’d say give Verbatim discs a go - LG drives can sometimes be picky about media quality, as well as the fact that it’s a Macbook/laptop drive, I think that’s where your problem is.
EDIT: Also, if those are 16x discs you’re using, firmware support for them on some slimtype drives might be lacking - if that’s the case, then it’ll use a “default” write strategy that may or may not be very good. Firmware updates for these drives may also be pretty scarce. With decent media like Verbatim, firmware support is more likely to be there for them.
Good luck! If all else fails, decent discs should fix the problem.