I have 12GB in my system and wanted to add 4GB more so I got one. But the system does not recognize the memory and shows slot as empty. I have 4GB 2Rx4 PC3-8500R-7-10-JP (3 of this) and the new one I got is 4GB 2Rx4 PC3-8500R-7-10-E1. the only difference is E1 and all others are same. Is this the problem? or do I have defective memory?