I know one of the main differences between past perfect progressive and past progressive is the fact that the latter can be used when the duration of an event or its exact timing is not important to us. In fact, to me, it's like a huge help, making the sentence construction more simple.
Now Consider these:
- The computer was working flawlessly for two hours before you inserted that disk.
- The computer had been working flawlessly for two hours before you inserted that disk.
- If we can use an adverb of time as in #1?
- Do they mean the same thing?