I just bought a Samsung 840 120gb SSD. I've plugged it into my mobo (a fairly old Gigabyte EP31-DS3L), and I can see the device in the BIOS. Also, the first time I booted windows 7 after plugging it in, a little plug and play hardware install thingy popped up in the bottom right of the screen and it correctly identified the drive. But even after multiple reboots it's not appearing in the list of drives on the computer and I can't access it.
The really annoying thing is, I can see it from within the Samsung "Magician" software which came with the drive. I can also see it in the list of drives in the windows "diskpart" tool. But I can't actually read it as a hard drive within Explorer.