Can I fit a second disk drive
I want to fit a second drive but I only have one socket on the motherboard
The XPS 420 has 5 SATA sockets on the motherboard, labelled SATA0 ... SATA4.
Your primary HDD should be connected to SATA0. A secondary hard drive should be connected to SATA1. Other devices such as optical drives should be connected to the remaining SATA sockets.
SATA0 and SATA1 sockets are located next to the RAM slots, toward the rear side of the computer.
The other SATA sockets are a few inches below.
The computer has 2 slots ("bays") for hard disk drives, near the bottom of the enclosure. One slot is occupied by the existing primary drive. The other slot should be available for an additional drive, and it should have a power connector already in place.
To fit a standard 3.5" HDD in the second slot, you need a mounting bracket. This is the blue piece of plastic that allows the drive to slide and snap in. It may already be factory fitted in the empty bay. If you don't have one, you can get it as a spare. Amazon probably has them. Search for "Dell XPS 420 HDD bracket". You also need a SATA cable, with a straight plug on one end and a right angle plug on the other end, about 2" long.
The user manual for the XPS 420 is available here. The procedure for adding the second hard disk drive is on page 132.
Here is a link to the Owner's manual for your desktop.
Scroll down to p.103-104, it shows that there are 5 sata ports on the motherboard. (#3 + #6 on the motherboard layout)
If you want to fit a 2nd HDD there are at least 3 spare ports available for you to use, if your current arrangement is only 1xHDD + 1xODD, unless you have more sata connected devices installed.