That's why plausible deniability should be implemented in such a way that one could have multiple encrypted profiles on a phone, each one with its hidden and encrypted storage, but the safe one wouldn't be empty or unused to avoid raising warning flags; all sensitive social media and communication would take place only on other accounts, still the user should maintain their day to day communications (family, friends) on the safe one. Encrypted profiles would activate only after entering their unique password, but the phone would immediately default to the safe one at each sleep or power off. This however doesn't protect from cell traffic analysis which would reveal for example the phone (tower+location+source and dest. IP+IMSI+IMEI+time) was connected to that server at a certain time, still would require more efforts from an officer than merely scanning all mail or posts on social media. A VPN helps, but as per above, now one would have to explain why they're using one, and I totally expect VPNs to be criminalized very soon.

If you use a SSD or above, you do not have any control over these "blocks" (called cells in these technologies). You cannot access them at all once they are unliked (i.e. the slack source you had with HDD which you could access does not exist anymore).