The 420 is designed for 3 internal HD's with no problem, just read your manual.

 

Two HD's go in the normal bottom HD bays, and the manual describes how to mount the third HD in one of the top 3 1/2 drive bays..

 

Just remember, their are power suooly cables already in place for HD's 2 and 3, but you'll have to supply your own data cables. The data cable for the bottom HD needs to have a right-angle connector on the HD end, while the data cable for the 3rd HD mounted in the 3 1/2 bay just takes a normal cable with straight ends on both ends of the cable.

 

If you need more than 3 HD's, you should be able to use some 5 1/4 to 3 1/2 drive mounting adapters and mount another HD or two in any un-used upper 5 1/4 drive bays.

 

 

what TimD said with one caveat, this assumes you haven't already used both 3 1/2" bays.... BlueTooth 13 in 1 takes up one 3 1/2

a floppy would take up one 3 1/2

the accelerator "might" take up one 3 1/2 - Not sure since it might be only using the front facia which you wouldn't need for a hdd....

 

The other option is to use the esata or add a pci card with additional esata ports...

eSATA would be an external drive (e for external)... similar to this one:  http://www.newegg.com/Product/Product.aspx?Item=N82E16822148237  (there are dozens of others, that just one example)

 

PS: but dig thru these threads about a known issue w/ the 420's eSATA (best to wait and make sure it's fixed before investing in an eSATA drive)   http://www.dellcommunity.com/supportforums/board/message?board.id=Tech_Talk_XPS&thread.id=26761

The 420 already has an eSATA port.

I can fit 4 in my 410 just put them in the drives under the DVD drives.

 

 

 

 

 

 

XPS 410
Intel C2D E6700 2.66
2 GB @ 667mzh (2 dimms)
Dual 80GB Raptors @ 10,000 RPM in RAID 0 (silenced)
320GB hdd
160GB hdd
NVIDIA 7900GS Zalman VF-900cu
ATI TV Wonder 650 HD
Diamond 24-bit 7.1 surround, w/ dolby digital live
2407 Monitor
LG 42" 1080p
logitech G7 mouse, G15 keyboard

----------------------------------"GAME ON"-------------------------------

The 420 was designed to mount 3 hard drives.

 

Two at the bottom of the case in normal plastic HD carriers.

The manual specifically describes how to mount a 3rd HD in the upper 3 1/2 drive bay at the top of the cabinet. ( I have my WD 1TB drive mounted there.

 

Other than that, you can choose on your own to mount additional drives in any un-used 5 1/4 drive bay at the top of the case if you buy some 5 1/4 to 3 1/2 drive mounting adapters and mount them yourself. It's easy to do, but mounting HD's in these locations is entirely up to you.