On a more serious note, as others have said, you'll probably burn through these weird storage limitations quickly.

Also, what do you mean by "sensitive matters" on Mint? Because almost any way you spin it, I feel like it's not a great idea:

• If you're talking professional, confidential work with clients, keeping it on the same device where you do anything personal sounds like a terrible idea, and it's probably worth it to shell out for a dedicated device just for this.
• If it's more personal things like government documents, medical records, and other things I'll neglect to name here, running a separate operating system just for those just feels like unnecessary paranoia and will cause you unnecessary trouble. If you're careful, it shouldn't be a problem - the major browsers prevent file access through protections against cross-site scripting.

Also, as I said in another comment here, please upgrade that drive before you put a lot of data on it. If you don't and you run out of storage later (a near-certainty on 256GB), you'll have to go through the effort of getting everything copied, which may include equipment purchases and several hours of your time when you could jut do it right now while your important files are still small enough to fit on a flash drive right now. Save yourself the future trouble.

Anyhow, I wish you happy Linux usage.