In case you are one of the three people on the planet that this would affect:
The first time you activate an earlier Xsan volume on macOS Sierra, the Xsan file system manager (fsm) upgrades your volume’s metadata to a newer format. Once this conversion begins, you won’t be able to use this volume with Xsan MDCs running OS X 10.11 or earlier.
Older MDCs won’t be able to control Xsan volumes that are created in Xsan 5. If you attempt to start a volume that has been upgraded to or created in Xsan 5 on an older MDC, the volume won’t open. When this happens, the system will deliver an ICB alert.
Here’s what Xsan is.