QNINE 2nd HDD SSD Hard Drive Caddy Tray Replacement for Dell Latitude E6400 E6500 E6410 E6510 Precision M2400 M4400 M4500, CD/DVD-ROM Optical Modular Bay Adapter to 2.5 Inch 9.5MM with Bezel;

The Hard Drive Bay Adapter gives you maximum storage flexibility.
enabling you to add a second ThinkPad notebook Serial ATA hard disk drive into the Ultrabay media bay of systems
supporting Serial ATA drives.
With the second HDD in your Ultrabay, you have the fastest way to back-up key applications and critical data.
Compatible Hard Disk : 2.5" 9.5mm SATA Laptop Hard Drive
HDD Interface: SATA
Machine interface: SATA
1. Fast & Reliable connection in laptop, just as the primary HDD
2. Fully integrated into laptop, no cables/PSU/software driver needed
3. Dual boot, Support installation of 2nd operating system on the 2nd HDD and boot selection
Great for backups or if you just need more space! Another thing you could use is to have a complete image of your main hdd (which has operating system), you always have a peace of mind knowing that you have a complete good working HDD backed up, whenever the main drive is infected with virus or spyware, you could take the back up and swap with main drive, do a complete image of it, then swap again, no need to re-install the OS and all applications.
Compatible with DELL Latitude E6400,E6500
Precision Mobile M2400, M4400