I have a problem with my pc dell t3400 also have two harddisk 1 is not detected why

YOu can remove the battery of your bios it will reset your bios password.

http://www.manualowl.com/m/Dell/Precision-T3400/Manual/187799?page=96


SAME HELP.

WIndows CD is the most likely method

If this desktop can boot to BIOS properly, then you can download the BIOS file from Dell's website, make a bootable media (using other computer) and use this media to update your T3400 BIOS.

Some body can help me please

you need to scan your pc with an updated antivirus

its possible to get data from harddisk of the pc, make sure you have connected power to harddisk and its data cable to your laptop so that it will be detected

Turn the machine off
Open the case
Look to see if there are IDE connector (Wide Flat cable with a connector usually grey)
If this is not present look for a SATA connector usually a small red or blue wire similar to a cat5e network cable
Scew your HDD in place in the rack connect the cable either IDE or SATA
If you have an IDE drive look for a 4 pin female connector from the power supply
If you have a SATA drive look for the connector from the power supply
Once all is connected then reboot you computer typically your bios should auto detect it.

Let me know what happens from here

check in bios your harddisk is detected or not.
if your harddisk is not detected then u have open cpu then replug power and data cvable connection to your harddik and mother bord again.
then u can detect harddisk.
it is issue of loos connection betweeen harddisk and motherbord.
check it .

try a different SATA power connection, sometimes they work loose and this issue occurs.