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The OS writes files by going (approximately) "create/open entry for file on drive's index, stream data to file from program, write details (e.g. blocks used so far) to drive's index, repeat previous two steps until program says it's done or the program says it's encountered an error or the drive runs out of room or insert-other-condition-here, write final details in the index and close entry for file". Or in even simpler terms: at the system level all files are written one block at a time, no matter how many blocks they'll eventually involve. Now a workaround for programs that deal with fixed file sizes is to ask the OS in advance "how much free space is on drive X" so that they can know whether there's going to be room before they start writing (well, "know" as in "guess" because other programs might also write to the drive and then it becomes a competition). But the catch there is that when the OS in turn asks the drive pool "how much free space do you have", DrivePool reports its total free space rather than the free space of any particular physical drive making up the pool. This is because it can't know why it's being asked how much free space it has (DP: "am I being asked because a user wants to know or because a program wants to write one big file or because a program wants to write multiple small files or because some other reason, and oh also if I've got any placement rules those might affect my answer too?" OS: "I don't know").