This is very much possible that we have free storage space but still we cannot add any new data in the file system because all the Inodes are consumed as the df -I command will show that. This may happen in a case where the file system contains a very large number of very small-sized files. This will consume all the Inodes and though there would be free space from a Hard-disk-drive point of view but from a file system point of view no Inode available to store any new file. A storage unit can contain numerous small files. The inode structure fills up before the data storage of disk, no more files can be copied to the disk. Once inode storage is freed up in the structure, new files can be written to storage.