DFS used a very simple disk structure and files on disk were located only by their length and starting sector. This meant that all files had to exist as a continuous block of sectors and fragmentation was not possible.
Sometimes a user would have a disk with a data file on it, and would create another file which DFS would place in the free space following the existing file; or a badly-designed program would open a data file and then itself create another file which DFS would place in the following free space. In this case, appending to the data file would succeed only until it grew as far as the beginning of the file following it. Regardless of how much free space might remain on the disk in total, it was not available to extend the data file, and the error message Can't extend would be issued.
Standards of error handling at the time were primitive and in any case programs squeezed into the limited memory of the BBC Micro could rarely afford to waste space attempting to handle errors gracefully. Instead, the user would find themselves dumped back at the command prompt with a blunt Can't extend and all the data which had yet to be appended to the file would be lost.
The resulting frustration would be only the greater if the user had taken the trouble to check the free space on the disk beforehand and found there to be plenty of it. That while it might exist it was not in the place where it was needed was not apparent without mentally analysing the screenful of numbers presented by the disk catalogue and so would escape the user's notice. In addition, DFS users had almost without exception previously been accustomed to cassette file storage, which does not suffer from this error. The upgrade to a floppy disk system was expensive, but it freed users from the unreliability and terrible slowness of the cassette system, and it was a shock to make the sudden and unpleasant discovery that the upgrade which was otherwise so wonderful might without warning cause the total loss of all their recent work.