Cant "drag and drop" to dvd, but to cd works ok

until recently I could “drag and drop” to both cd and dvd but now it only works with cd. when trying a dvd I get the error “disc write protected, take protection off or use another disc” and when I try to acces the same dvd again I get this error "Disk not formatted, windows cant read this disc, it may be damaged or using a format not compatible with windows.
I have spend about 25 new dvd different brands which now all seem too have error.
I use:
winxp home
toshiba sd-r5002
instant cd/dvd VOB


Tried to make a multisession disk with Nero: dvd-rom(udf) and according to NERO the burnprocess completes succesfully, but once again when trying to acces the dvd I get the error:
Disk not formatted or damaged. I could acces the dvd before burning it.
I can also acces dvd,s I burned exactly the same way before this problem started

:confused: :confused: