I have a very similar problem to Matt's problem here. I had 2 Barracuda XT 3TB SATA drives in my desktop PC running Windows 7 Ultimate 64-bit. I sold the PC but kept the drives and bought a USB 3.0 enclosure so I can access my data from my laptop. The laptop is running the same version of Windows 7. The computer recognises the external drive but it does not appear in Windows Explorer. Windows Disk Management says I must initialize the disk.
I have tried it with both 3TB drives from the PC with the same result. I tried it with a new drive and it works fine so the enclosure is not at fault. I also tried them in another enclosure with the same result.
Any ideas? Will I lose everything on the drive if I let Windows initialize it? Matt doesn't let us know what happened in the end with his drive...
PS The desktop PC had an Asus P6T Deluxe motherboard and used the latest Intel Matrix Storage Manager.