FDB refuses to process writes (degraded write availability) when usable disk space goes below 10%. Nothing will be lost though.
Basically, fdb is great as long as you avoid this situation. If you do, woe unto you trying to being the cluster back online by adding nodes and hoping for rebalancing to fix things. It will, it is just very, very slow. I don't know if that's true in the current version.
[0] https://forums.foundationdb.org/t/brand-new-macos-installati...