I have 2 hdd but ony 1 shows in my cpomuter?

First, is it visible in BIOS?
(DEL or F2 on the first screen, when the memory is tested, then General, watch the screen for hints)
If it's not visible there, it means one of few things:

1) Wrong jumper settings (if it's ATA) - one disk needs to have jumpers set as 'Master' (and be placed at the end connector of the tape) and the other has to be 'Slave' (and be attached to the middle connector). Refer to the plate on the disk for the right jumper settings.
2) Connector not plugged well enough. Loose tapes are 50% of cases of all problems.
3) It's incompatible with the other disk. Happens rarely, but happens. You may get a second tape and attach it to a second IDE channel (usually just by where the first tape is plugged in)
4) Also, whether it's IDE (a wide, some 2inch ribbon connector) or SATA (a narrow, half-inch red roundish ribbon) it can be still switched OFF in BIOS. Look for 'ATA controller' or 'SATA controller' in BIOS and switch it on.
5) Oh, and check if the power line is plugged in as well.

If it's visible in BIOS, likely it's not formatted. Partition and format it using the right software in Windows.

be sure to properly check jumper settings then check the system BIOS settings.

Have you just installed your HDD, did you initialised it?

If not, right click on my computer, then select Manage.

On the left of the Computer Management window, click on Disk management. If you have installed the disk correctly, there should have a disk listed on the right that hasn't been initialised. Right click on that disk and select "Initialize Disk".

If you can't find the new disk, there is a possibility that you didn't install the disk properly. Switch off the computer, and check if the HDD is physically installed correctly, after that turn on the computer and go into BIOS to check if you have two HDDs attached.

problem no 1: the cable of ur hard disk may be not attached properly.
problem no 2: may be ur hard disk is in ntfs and ur using old version of window so it may not show there
problem no 3 : go to bois and see if its shown there or not

go to control panel . click administrative tool: then click computer management : then click disk management. there u can see those disk which usually hided.
and if u cant see that disk there then some problem in ur hard disk

If you're running windows xp open the control panel and double click on administrative tools
click computer management
in the left hand pane towards the bottom of the list click disk management
you should now be able to see your missing hard disk
right click on it and click format
after formatting it should show in my computer

Did u check the jumpers switch? Is it set correct?