This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
Disk Manager 6.x and above is a piece of software that performs the translation necessary to access harddisks of more than 1024 cylinders with DOS/Windows. This is achieved by installing a Dynamic Drive Overlay (DDO) to translate drive parameters. The driver provides only the basic translation functions, without EDPT or extended int13 calls. Of course, this is less of an issue in a software driver than in a system BIOS.
If Disk Manager (DM) is used to format only the slave drive, the DDO can be installed in the config.sys as an ordinary device driver (device=dmdrvr.bin). On the other hand, using DM on the master drive isn't quite that easy from a technical point of view. Since you must boot DOS from the master drive, and you must load the DDO before you can access the drive, the only option is to load it very early during the boot process, even before the operating system itself. Changes are made to the Master Boot Record (MBR) to accomplish this 'pre-boot loading'.
This scheme works fine, but has a few drawbacks.
o First, DDO as a pre-boot loader has implications for floppy boots. If you boot directly from a floppy, DDO does not have a chance to boot and the partition does not make sense. This can be remedied by having a line in the config.sys on the floppy which reads "device=dmdrvr.bin". You can also watch for the press spacebar to boot from floppy message when booting from the hard drive.
o The second drawback is that operating system installations routinely overwrite the MBR with their own boot code. The DDO is no longer loaded and your partition will be inaccessible until you let Disk Manager write a new MBR.
o Third, a nonstandard partitioning scheme is used whereby the data is offset by one track. This is completely transparent as long as the drive is accessed through the DDO; however, many operating systems want to replace the DDO by their own disk routines and will have to be aware of this scheme.
Windows 95 will support Disk Manager 6.x and above 'out of the box', as will new versions of OS/2 Warp, Windows NT 3.5.1 and Linux 1.3.x. IBM and Microsoft have created fixes that allow older versions of OS/2, Warp and Windows NT to work with Disk Manager. o Fourth, corruption of the DDO sector will result in a DDO Integrity Error. While it is fairly easy to re-write the DDO sector (use the dmcfig.exe utility on your DM diskette), this is is a sign of a bigger problem (eg. virus) rather than a problem in itself---contact Ontrack tech support (firstname.lastname@example.org) for assistance.
An advantage of formatting the master drive with DM instead of loading the DDO from config.sys is that you can use Windows for Workgroups' 32-bit file access on both drives---if you use dmdrvr.bin, the slave drive is restricted to 16-bit file access.
The 6.x versions of Disk Manager have some additional disadvantages which are corrected in version 7:
o They are not fully compatible with the device drivers of most VLB ATA(-2) interfaces; also, ATAPI CD-ROM and tape devices on the chain are not supported.
o A final concern is disk utilities. If the utility in question goes directly to the hardware, without going through the DD overlay, it can potentially be destructive. Ontrack's policy on this is to refer compatibility questions to the manufacturer of the utility as they cannot possibly maintain compatibility charts for all versions of all utilities.
You can find more information on the various versions of Disk Manager on OnTrack's www site <http://www.ontrack.com>.