This article is from the Apple II Csa2 FAQ, by Jeff Hurlburt with numerous contributions by others.
A "disk image" is typically a file containing every data byte on a diskette-- i.e. Catalog tracks, files, DOS (if present) etc.. One kind of disk image, NIB, tries to preserve all disk information (e.g. sector headers, sync bytes, etc.). Apple II emulators running on a PC, Mac, etc. treat disk image files like diskettes. Disk image files are also a handy way to archive Apple II disks on hard disk and to maintain wares on ftp and other download sites. DSK's (.dsk, .do, .po and .hdv files) DSK (usually .dsk) files are disk image files used by popular Apple II emulators like AppleWin to run A2 wares on the PC or Mac. Usually, they are images of Apple 5.25" game, utility, etc. diskettes. A standard 5.25" DSK file is 143,360 bytes in length: 1 side x 35 Tracks/side x 16 Sectors/Trk x 256 Bytes/Sec = 143,360 Bytes. DSK files of 800k 3.5" disks are much less common. Data in a DSK disk image file can be arranged in the sector order used by DOS 3.3 or in the sector order used by ProDOS. The filename suffixes relate chiefly to how data is arranged in the file: .dsk- technically, this may be an image which has its data in DOS 3.3 or ProDOS order. (The emulator program is supposed to check a .dsk file to determine the ordering used.) It has become standard practice to use the .dsk suffix for only DOS 3.3 order files. .do- an image which is in DOS 3.3 order. This suffix is seldom used today. DOS 3.3 order image file names usually end with ".dsk". .po- an image which is in ProDOS order. If an image is in ProDOS order, its name should end with ".po" (not ".dsk") to avoid confusion. .hdv- typically an image 800k (819,200 bytes) or greater in size in ProDOS order. The image is intended for use as a virtual hard disk by various Apple II and IIgs emulators (e.g. Apple Oasis). The IIgs program ASIMOV2 can create .hdv files (select "Raw image"). The file name should end with ".hdv". Note: data order does not relate to whether a disk image is a DOS 3.3 or ProDOS disk. In fact, nearly all 5.25" disk image files (of both DOS 3.3 and ProDOS disks) are in DOS 3.3 order; and, DOS 3.3 order is the default setting for image creation programs like DSK2FILE and ASIMOV and the transfer/creation program ADT. On a PC, NuLib can create disk images from 8-bit ShrinkIt whole-disk (.sdk) files (but not from .sdk files which were produced by GS-ShrinkIt). These images will be in ProDOS order. You can convert a .po disk image to a DOS 3.3 order .dsk by using a disk copier like Disk Muncher on an emulator to copy from the .po image to a .dsk image. On ftp sites, DSK files are usually in a ZIPped form to conserve space. For example, on the Asimov site, narfgames.dsk.gz is a DSK file of the narfgames disk which has been g-zip compressed. Other archive sites may use standard ZIP compression and the file name might be "narfgame.zip" or "narfgame_dsk.zip". On a PC, WinZIP will uncompress g-zipped and ZIPped DSK files. A DSK file can be converted to actual diskette form on an Apple II using DSK2FILE or (GS-only) ASIMOV. If a 5.25" .dsk disk image file is transferred to your Apple II using ADT (or ADTgs for IIgs), it is automatically converted and written to 5.25" diskette. For more about ADT and ADTgs see Telecom-1. Most 5.25" DSK (.dsk and .do) files are of a DOS 3.3 or some related DOS disk. The target diskette should be INITed for DOS 3.3. (or, it can be formatted using Copy II Plus, etc.) and you should use the default DSK2FILE or ASIMOV "DOS 3.3 Order" setting. If a disk image file has a .po suffix, use the DSK2FILE or ASIMOV "ProDOS Order" setting. Note: In most cases it is okay to use either a DOS 3.3 or ProDOS formatted diskette as the target (and; the target disk does not need to be empty of any files). However, ProDOS formatting uses a default Volume Number of 1, which is different from the DOS 3.3 default of 254. Since ProDOS stuff does not care about Volume numbering and DOS 3.3 stuff may, the target disk should generally be one INITed with the default Volume Number-- e.g. INIT HELLO . Here is a quickie step-by-step guide for getting a 5.25" DSK disk image file into useable form: 1. Download the file in binary mode from an ftp archive site via ftp:// ... 2. If file length is not 143,360, use WinZIP or equivalent to unZip it. 3. Transfer the DSK file to your GS via Mac diskette or a NULL modem transfer. One way or another, the file needs to end up on a ProDOS diskette or ProDOS hard disk volume on the GS. 4. If you are using DSK2FILE, jot down the complete path name of the DSK file (e.g. /RAM5/NARFGAMES.DSK ) because DSK2FILE will ask you to type it in. 5. Insert the formatted 5.25" target diskette into Drive 1 (Slot 6). This diskette needs to be 16-sector formatted. Plain DOS 3.3 formatting with the default Volume number is, generally, best and easiest. (You can boot a DOS 3.3 or Prontodos disk and do an INIT HELLO to format a 5.25" diskette.) 6. Start DSK2FILE or ASIMOV. Normally, you will accept the defaults (5.25", DOS 3.3 order). If you know the DSK is a ProDOS image in ProDOS order-- like the file name ends with ".PO", select "ProDOS Order". (ProDOS disk images are, fairly often, in DOS order to make them more universally transferable.) 7. Select the "Image file ---> Diskette" option, follow prompts, and you should end up with a good diskette. (If everything seemed to go well but the disk does not work, try repeating the process using the other "Order" option.) DSK2FILE and ASIMOV can, also, create disk image (.dsk or .po) files. Similarly, using ADT to transfer a 5.25" disk automatically creates a .dsk disk image on the PC. The source disk can be for a game, etc. so long as the diskette is not copy protected. Note: DOS 3.3 products which depend upon Volume numbering to identify diskettes will normally not work in disk image form on an Apple II emulator because Volume number information is embedded in non-data parts of a disk and is not included in a standard .dsk disk image file. NIB (.nib) Some copy protected diskettes can be converted to another kind of disk image called "NIB". Saltine's Super Transcopy (SST) incorporates bit copy routines to attempt to produce a nibblized disk image of a 5.25" diskette. On your Apple II, SST reads the disk bytes from half a disk and stores that data on a whole normal disk. Then it does the same for the second half. These two disks can be converted to .dsk disk images and moved to a PC or Mac. There, the .dsk images are merged into a NIB image using SST running on an emulator. If successful, you have a .nib file which can be used like a diskette on popular Apple II emulators. (For one or two older emulators, .nib files are the only useable images.) The standard length of a .nib file is 232,960 bytes-- much larger than a DSK. However, since .nib files include sector address header and other non-data 'embedded' diskette information, they can be used to image many protected disks. Naturally, a .nib file preserves DOS 3.3 volume numbering. This allows programs which use volume numbers to identify their disks to run on emulators. Many disks with no copy protection are in .nib form instead of .dsk because the game, etc. which uses the disks needs to check volume numbering. 2MG (.2mg; sometimes .2img) Today, more and more IIgs software is being converted to 2MG disk image format used on XGS and other IIgs emulators. These are .dsk or .nib images with a prefix (usually 64 bytes) which includes information about size, format, sector ordering, volume number, locked/unlocked, etc.. 2MG files may also have a Comment and/or extra file information added following the disk image data. The format can accommodate disk images ranging from 5.25" diskette up through hard disk. For 2MG format details, see http:// apple2.org.za/gswv/a2zine/Docs/DiskImage_2MG_Info.txt . The usual length of an 800k .2MG image (with no Comment or extra data) is 819,264 bytes*. You can use ASIMOV2 to convert .2MG files back to diskette form as well as for creating .2MG files from 800k diskettes. The utility Imgutnew.exe can be used to convert most available Diskcopy images to 2MG format on PC. *See ... Size Note: Transferring to 3.5" disk (at bottom of this page) By: Rubywand