i too ran into the problem of the DH-20A4P suddenly not seeing DVDs anymore
I ruled out any OS problems
it would not see DVDs to boot from
using the LtnRPC.exe tool i reset the region to 0 (user and device)
then reset to 1
now it sees dvds (video and data) in win enviro. also it will now boot from DVDs
(and i was about to order a replacement!!)
hope this helps someone else