I'm writing a story where a security expert has been kidnapped and tasked with gaining access to some files on a stolen computer protected by full-disk encryption, and I'd like some ideas for ways he might accomplish that. The encryption is software-based, and the character has physical access to the entire computer.
The people that kidnapped the character don't know who has the encryption key, so the $5 wrench method is out. The computer has not seen use for a very long time, and is only used occasionally (on the scale of once every couple of years), so introducing a keylogger or some other method that relies on capturing user input to find the key won't be useful. The character knows that he's going to be killed if he doesn't manage to get access to these files, so he's pretty motivated to try anything that might work, even if it would ordinarily be too tedious or fiddly to bother with. He has about a week to produce results.
The computer in question is an alternate-universe system basically equivalent to a 1998 PC, running the fantasy equivalent of Linux. The character is attempting to break into it with roughly 2008-level technology. He definitely doesn't have access to enough computing power to brute force the key in a reasonable amount of time.
So, what are some approaches the character might take? From what I've read about full-disk encryption, cold boot attacks are a method for breaking it if you have physical access to the machine, but since this one hasn't been used in a long time I don't believe that's an option.