FIVEC attempts to minimize problems around saving to incompatable formats by detecting which formats are valid to save to, given the disk parameters. Simply select the format you want from the lower listbox, and press the "Save" button. This will bring up a file dialogue for the name.
It may not be immediately obvious for those unaccustomed to the various formats why saving is unavailable in a given format. For example, this disk cannot be saved as FDI:
The reason for that is that the image is missing sectors, a feature which is not supported by FDI:
Of particular note as the BKDSK formats (XDF,HDM,HD4,DDB,DD9,BIN), which are completely headerless, and consist only of the disk data in sequential order. These disks are recognized only by the combination of their file size and the extension (FIVEC actually detects them for loading only by the filesize, and having ruled out other compatable formats.) FIVEC will automatically append the matching extension for these images to the filename when you save.