Of the tests I was doing on the DVDs, these were the results, using both a Sony DRX-800UL external DVD burner and the internal drive on my IBM ThinkPad R52 (Matshita DVD-RAM UJ-830Sx) and Nero 18.104.22.168.
When attempting to burn files (I used a directory with 1GB of files), the Sony drive consistently came back with “Illegal disc”, both when selecting 2x and 4x speeds.
The internal Matshita drive would only select 2x for the DVD speed, but burnt the files successfully.
Both drives can read the subsequently burnt disc but again, only the Matshita [internal] drive could erase the disc (it appears the Sony drive cannot write to it no matter what speed).
Another user with a Sony DRX-810UL-T (external DVD burner) also has the same problem (ie. the “illegal disc” error appears when using these discs as well).
Various internet forums suggest a firmware upgrade but to the average user, this is a rather daunting step (and personally, I’d have swapped the discs for another brand if I weren’t a technical user). In any case, the Sony drives do not have firmware updates available.