I've just run into the same problem with my DVDR-75 after almost two years of flawless service and hundreds of recordings.
In my case, I've discovered that newer burns on Tayo Yuden media (T02-00)
would sometimes produce the "unknown data" error, whilst older recordings
made on Memorex (MCC 003) never has reading problems.
Yesterday, I decided to experiment and see if it might be "media" related.
So I used my computer & DVD Decryptor and burned an ISO copy of the problematic disk onto the Memorex media.
Lo and behold....it worked!
The machine had no probs reading & playing the Memorex disk.
You might try the same procedure and see if it works for you.
ps. Strangely enough, after playing the Memorex disk, the ability of the
DVDR 75 to read & play the problem TY disk was seemingly restored!
I wonder if there might be a media-related laser power calibration problem that is intermittant?
I will soon do a few more tests and post my findings if anyone is interested.
Hope this helps,