Absolutely. Worst thing that may happen, although highly unlikely, is the new PC locking up when going through config+autoexec entries related to specific h/w (like a sound card). So you might want to REM those out beforehand. But again, highly unlikely.

Ah great konc I bet software piracy was a problem in the day of Dos if you had time all I think you had to do was plug in a hard drive and copy everything to the new hard drive and there you would have Dos and probably Windows 3.1 for a different Pc.

It's absolutely possible. Though for the older machines (386/486 era) it is important to have correct disk geometry stored in CMOS.
If the "new"machine has an auto-detect feature, this should be easy. If not, just write down the settings in old CMOS Setup and enter them manually in the new.
(Note: Some CMOS Setups do count differently than others. Ex. Cylinders 0-1023, Heads 0-15, Sectors 1-63)
After you're done, you can run SCANDISK or CHKDSK /F and see if the files are all good and accessible.
If something is wrong (misaligned), either of it should mentioned it during the scan.
Edit: See History of BIOS and IDE limits - http://www.tldp.org/HOWTO/Large-Disk-HOWTO-4.html

GabrielKnight123 wrote:

I think you had to do was plug in a hard drive and copy everything to the new hard drive and there you would have Dos and probably Windows 3.1 for a different Pc.

I agree. ^^ Hoever, it's better to not copy the Windows 3.1 swap file from one drive to another.
Reason is, this file is stored in a specific, fixed location on the hard disk. It's not movable.
Windows 3.1 does this, so it can access the swap file without requiring to use FAT or DOS.

The only time I've seen an MS-DOS installation fail to boot when swapping motherboards (excl. HDD geometry) is when moving from a 286 or better system to an 8086/8088 one. I assume it has to do with DOS making use of 286+ CPU instructions when installed on such a system, since going the other way around works fine.

If you use a Pentium or newer motherboard, then hard drives are interchangeable. But in the 486 era when LBA was not that mature, there can be troubles, that if you switch hard drives, they are unreadable.

GabrielKnight123 wrote:

Hi all, I have a Pc setup using just Dos 6.21 with Dos installed using 3 floppies and I was wondering if I have to use the 3 floppies again to reinstall Dos using the same HDD but with a different motherboard or can I just swap the HDD to the new motherboard and keep Dos as it is.

What are the "donor" PC and the "recipient" PC hardware configurations?
The DOS is not hardware-locked like Windows NT, only some third-party drivers in CONFIG.SYS and AUTOEXEC.BAT may be hardware-specific. But most of them simply says that "driver can't be loaded" and then system continues loading. Vanilla DOS have no hardware-specific stuff at all. You can simply swap HDD to any other machine and it will work exactly as on original PC.

But older BIOSes have some differences from modern ones. Modern BIOSes (since Pentium and later 486s) have HDD geometry (C/H/S) and mode (CHS Normal/CHS Large/LBA) auto detection. Older BIOSes are not too smart and HDD configuration should be entered manually in the CMOS Setup utility. Most 386/486 BIOSes can help in geometry entering, they have "HDD Auto Detection" tool that can automatically enter the geometry. But access mode (CHS/Large/LBA) still should be entered manually. Note that if the C: partition is larger than 500 MB the BIOS must support Large or LBA access modes. BIOSes without them cannot boot from such HDD until it will be repartitioned and reformatted to 500MB size.

If the HDD is a MFM or RLL disk it cannot be connected to any other controller without a low level format. It is limitation of ST506/412 interface. Only IDE and SCSI drives can be used separately from their original controller (or motherboard).

Windows 3.1 is very portable, but you must change video driver by hand if the video card on the other machine is not same as on the original. Unlike Win9x, 3.1 cannot fallback to VGA mode, it will crash or freeze on foreign video card. Change of the driver can be do via SETUP.EXE inside Windows folder, even when the Windows cannot boot due to video driver.

I moved winxp and winvista to new motherboard without problems too.

robertmo wrote:

I moved winxp and winvista to new motherboard without problems too.

XP and 3.1 are some different systems. 😀 NTs can be booted on other HDD controller (other IDE or other SATA) only when drivers for both controllers (old and new) are installed. DOS and Win9x are not locked to HDD controller drivers, so they will boot in any conditions where BIOS can see the boot partition. I.e. when the C: is visible from boot floppy, DOS/9x should boot too.