Problem with vobsub

[BluDChyLD]

Everytime I use vobsub to extract the subtitles from my vob files about half way through the indexing stage it halts and says that my dvd drive is either locked or there are missing or truncated files. This has happened with every dvd I have tried so ive stuck to using subrip. The only thing I have done to modify the vob files during ripping is extract the only audio track I need to save space. Could this be the problem?
Thanks,
BluDChyLD