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DFS used a very simple disk structure and [[Computer file|files]] on [[Hard disk|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. Using the example in the table above, the attempt to expand file F in step five would have failed on such a system with the ''can't extend'' error message. Regardless of how much free space might remain on the disk in total, it was not available to extend the data file.
{{Essay-like|section}}
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 the ''Can't extend'' message and all the data which had yet to be appended to the file would be lost. The resulting frustration would be greater if the user had taken the trouble to check the free space on the disk beforehand and found free space. While free space on the disk may exist, the fact that it was not in the place where it was needed was not apparent without analyzing the numbers presented by the disk catalog and so would escape the user's notice. In addition, DFS users had almost without exception previously been accustomed to [[Data cassette|cassette file storage]], which does not suffer from this error. The upgrade to a [[floppy disk]] system was expensive performance upgrade, and it was a shock to make the sudden and unpleasant discovery that the upgrade might without warning cause [[data loss]].<ref>http://www.8bs.com/hints/083.txt - Description of the ''can't extend'' error</ref><ref>http://8bs.com/mag/1to4/basegd1.txt - Possible data loss caused by the ''can't extend'' error</ref>
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