If your PCI-e USB 3.0 adapter has an NEC chipset, the problem may indeed be the driver. I had a similar problem with my WD 2 TB USB 3.0 HD not being recognized by my PCI-e USB 3.0 adapter. I came across an article explaining NEC was bought by another company, that the drivers for the NEC USB 3.0 adapters were faulty and the new company had no intention of updating the drivers at all.
I bought another PCI-e USB 3.0 adapter with a VLI chipset, and it installed perfectly, and recognized the WD external hard drive immediately, and has operated perfectly ever since.
If your USB 3.0 adapter uses an NEC chipset, that may be your problem right there. It was for me (I bought two of them actually). Get yourself a USB 3.0 adapter with a VLI chipset and install the updated driver 1.81 off the internet. That's my recommendation, as it certainly worked for me, and I already tried two different adapters that used the NEC chipset. The NEC drivers just don't work reliably on Windows 7.