Just a thought: Before you replaced the Optical Drive, did you change the drive letters manually?

Reply to GaRHaR: I do not have a card reader of any kind. In the rear I have a wireless mouse and a printer, and in the front the scanner.Thanks.

Reply to georgeks: I don't know because the replacement was done at the lab of the store where I bought the desktop. Right now the drives are ordered alphabetically from C to J. C,D,E,F are the parttions of one of the physical drives, H and I the partitions of the second HDD and G and J (this was the replaced one) are the burners.Thanks.

If I understand correctly, your problem is with the rear USB connectors. Yes?

Have you checked each of the rear connectors individually?
Do they all work if connected and tested one at a time?

Reply to Gandalf: all ports work fine individually, furthermore, any combination of two also work fine. The problem begins when I add a third device or more.Thanks

Reply to GaRHaR, georgeks and Gandalf: Additional information,when I connect a third device in the rear say a USB flash drive , I can see in Device Manager/Disk Drives the 2 hdd's and the USB flash drive for a few seconds. Then one of the hdd's disappears (always the same drive), the flash drive remains. Maybe this will give some hint. Thanks.