How do I fix the problem of my old hard disk drive...

Please don't do anything which would write to the drive. Your hard disk parameters in the CMOS setup on the new motherboard are probably not set the same as they were on your old computer. The new board is probably auto-detecting the drive and setting the parameters so they differ from the 486. My first guess is the 486 motherboard doesn't support Logical block Addressing (LBA) and Disk Manager or EZ Drive are installed on the old drive to implement LBA. Try to get the disk parameters off the 486 motherboard. You can get at the 486 by plugging in a power supply, CPU, memory, video card, and keyboard. It doesn't have to be put back in the case. Just make sure the bottom of the board is protected from shorting-out. Then go into the CMOS on the new motherboard and set the drive parameters so they are identical to the old motherboard using a user-defined drive. Once the old hard disk is operational, may then be possible to remove Disk Manager or EZ Drive and change the CMOS Setup to support LBA. The hard disk drive should be backed-up before attempting it.