Just finished the Dropbox loop last month, leveling for L5. Here's what the system design round actually looked like.
The prompt was open-ended: design a file sync system. Not exactly a curveball given what Dropbox does, but the depth they want is real. I went maybe 10 minutes on requirements/scoping before touching anything architectural, which felt right. The interviewer pushed back when I tried to over-engineer the metadata store too early.
Things they actually probed: How do you handle concurrent edits / conflict resolution. They wanted specifics, not vague CRDTs. Block-level deduplication vs file-level. Tradeoffs around storage cost. How the sync client decides what changed locally (inotify, FSEvents, polling fallback). Scalability for millions of concurrent clients, not just theoretical millions of files.
What I noticed: the interviewer seemed to care more about how I reasoned through tradeoffs than whether I landed on a specific architecture. When I said "I'd use S3 for object storage" they immediately asked what I'd do if S3 went down and whether consistency or availability mattered more for this use case.
The round is 45-50 minutes. Last 5-10 is for your questions, but they'll use that time too if you're mid-discussion.
For prep: distributed file storage, metadata services, vector clocks or operational transforms for conflict resolution, and client-side sync protocols. If you've touched anything Kafka or change-data-capture adjacent that's relevant because they care a lot about event streaming internally.
Leveling-wise, for L5 they want you driving the conversation. If you wait to be told what to explore next, that's probably going to read junior.