hard disk disappear

My friend told me that his disk (Quantum-3.5 series)installed Linux and winXP,but weeks ago he could not find his disk forever. I checked the CMOS,it cannot find the hard disk,even I changed IDE and jumper on the driver.The weird thing is when I connected the CD-ROM and the disk on the same controllers(the same IDE),CD-ROM cannot be found.But if CD-ROM on the second IDE or take out the hard disk, CD-ROM can be found.I don’t know the problem is in CMOS(virus or XP-lock)or HD,anyway I put my own hard disk on his IDE,it can recognised! His CPU is PIII-500,motherboard is AOpen MX6B/EZ.Who can help me to solve the problem?