I use an IBM THinkPad T22 for Amateur Radio applications' Windoze XP Pro with latest updates. The original memopy was 128 MB, upgraded with an additional 128 MB when I bough the machine. The service manual says it will take a maximum of 512 MB, and the modules listed state SD-RAM, PC-133 as upgrades. I purchased a 512 MB SD-RAM PC-133 module through Amazon, and it won't work! If I install the single module, I just sat a series of three beeps, with no boot up. If I add the module with one of the original modules, I don't even get the beeps. I noticed that the original memory (ine original from IBM, and one from INfineon, were both PC-100 modules.l Do the SD-ROM modules need to be installed in pairs? Or is the 512 MB module I received defective? I realize even 512 MB is barely enough to support XP, but it's far better than only 256 MB.