Not sure that is the problem, though the filesystem is fully allocated there's still space for metadata, this problem is more serious if it is fully allocated and the metadata is almost all used up since there's no space to create a new chunk, still run a balance because it's not good to be like that:
https://lime-technology.com/forums/topic/62230-out-of-space-errors-on-cache-drive/?do=findComment&comment=610551
P.S. disk1 needs a new SATA cable.