4GB PC3-10600E 240 pin DDR3 SDRAM 1333MHz Dual Rank x8 CL9 ECC Unbuffered UB DIMM Server Memory for HP. Parts-Quick Equivalent to HP Part Numbers: 500672-B21; 500672-S21; 537755-001; 593923-B21. Compatible with: HP Workstation Z400. THIS MEMORY WILL WORK ONLY IN THE SERVERS LISTED HERE. DO NOT BUY FOR ANY OTHER DESKTOP OR SERVER. IT WILL NOT WORK IN STANDARD DESKTOPS WHICH USE NON-ECC MEMORY. Some servers take both Unbuffered memory and Registered memory. However both types cannot be mixed. This is Unbuffered memory. Please make sure you need Unbuffered memory before buying this item. System Specific memory backed by Parts-Quick Lifetime Warranty and Toll Free Technical Support.