There are two parts to the issue:
You must contact Dell support by phone to have a master password generated - with this you can remove the hard drive password stored on the system board.
However, this will not remove the password from the drive itself. If you do not need the data on the drive, the cheapest thing to do once the CMOS password is removed is to simply replace the drive with a new one.
If you DO need the data on the drive, the you will need to send the drive off to a data recovery service to have the password removed and the data retrieved. This will be a minimum of a few hundred dollars -- if you don't need the data, replacing the drive is cheaper.
Note: if you attempt to install a new drive with the password still in CMOS, the new drive will inherit the password. You MUST get that cleared by Dell support.
When the HDD password is set the drive is locked on the next reboot. Even if you place the drive in another machine, it is still locked. Each drive has 2 possible passwords: user and master. Either PW can be used to trigger a security_erase_unit command from the ATA security command set.
Security erase will erase the drive and it will be unlocked after the next reboot. There is a program by Sanderson Forensics in The UK, called BXDR. You can use it to erase and unlock the drive. The CMOS component of the HDD password is probably cleared by clearing the CMOS chip (BIOS), either by removing a jumper cap or disconnecting all power and removing the CMOS backup button cell on the mother board. for a half-hour or so.
Beware, certain BIOS send a security_freeze_lock to the hard drive on power up. This prevents changes to the security command set functions of the drive. In this case the drive must hot power cycled to send any other security commands--such as security_set_password or security_erase_unit, to the drive
The center paragraph above is faulty information - the password is stored in EEPROM, not flash memory - it can be cleared ONLY with a master unlock code and the only legitimate way to obtain that is to call Dell support and verify your ownership of the system.