alt Hacker NewsI could be wrong, but my understanding is that the dummy OS views the remaining space as legitimate and accessible free space. Using dummy directly is of course dangerous, as you might overwrite sectors with legitimate data, but also, you can access dummy os from secret. so you'd drive dummy from secret to prevent that but can load dummy as main if under duress and it looks fine. Browser, logged into various inconsequential things, random files for inoffensive memes and other human stuff in downloads folder etc. maybe an email account you've signed up to a few newsletters and e-stores that send spam logging in via an email client that auto-launches etc.
Done well, I see no reason it should raise redflags in routine stops, so unless you're being targeted (at which point you've got way bigger problems) it should just seem like you're a run of the mill person who does not use their device to its full capacity, which is the majority of users.
at some point, having any mitigations even present is a problem. At some point being met with a boot password at all is a problem that puts you on a list. I have no solution there other than to not go to those countries or keep dummy hot.
If you read the linked thread, you'd see the reasons are:
1. SSDs (including phones) have TRIM/discard, so you need to disable it, otherwise the hidden volume would get wiped. You going out of your way to disable it is going to be suspicious.
2. Even if the above wasn't an issue, you can't really use the outer os to any meaningful extent, because you run the risk of overwriting the inner volume. That makes your decoy os suspicious. It's not definitive proof you have a hidden volume, but I doubt the authorities would care too much about that.