There may be several reasons for such a module to be recognized falsely: - mainboard chipset: for example, i44BX may have three slots, but the center one would not accept double sided architecture or the inner architecture of the chips requires to be handled as two banks, also not working on the inner slot, nor on one of the outer, with the inner occupied in any way - module chip layout; again, depending on chip's inner architecture, the mainboard may fail to work with 256 MB modules with only four or with 16 chips on it eight chips may then normally work - bad / old mainboard BIOS - another RAM module inside, that blocks resources try the 256 MB alone, first slot - badly programmed SPD EEPROM, try to disable auto detection in BIOS setup ... Don't take the single sided / double sided aspect to serious, as even if both sides of the module are equipped with chips, they may still be electronically arranged as single sided / one bank, and vice versa. In many cases, though not always, four chips or sixteen chips are double sided / two banks, eight are single then. Of course, only talking single data rate SDRAM here.