EOD Data that I download everyday comes as a single CSV file with one line per stock. This format is efficient for sharing daily changes and all exchanges disseminate in this format. However, Most charting software need data to be organised as one file per symbol and new data is to simply be appended to the file. Metastock is one such format. Amibroker has its internal format too but I do not use that.
Data from servers is directly converted and stored into Metastock format. Once that is done, same data file can be read by Gannanalyst, Amibroker, Metastock and Advanced Get without any need to make any further conversions.