This DisplayString reports the space actually available for use (excluding any reserved space) on the specific disk identified by multiDiskIndex, mirroring diskFreeAvail but per drive. As with the single-disk version, this can be lower than the raw free-total figure if the filesystem reserves space for privileged use, which matters for correctly predicting when a non-privileged logging process will actually hit a write failure on this specific disk. An admin should trust this available figure, not multiDiskFreeTotalBytes, when forecasting when logging on this particular drive will fail.