Windows 7: Why WD 3TB USB MyBook works, 3TB via USB-SATA adapter doesn't

I'm looking for help in understanding why my external 3TB WD My Book is working just fine with Windows 7 Pro 64bit. My PC is a Dell Precision 690, and only supports USB 2.0.



I have a new 3TB WD Green "internal" drive. Plugging it in to the PC I hit the "only 746GB seen" issue one can Google about. I expected this. The BIOS doesn't support larger than 2TB. But when I plug the drive in via a USB-SATA adapter, I still only see 746GB. I've been using these types of USB-SATA adapters for years. Internal drives are cheaper and I never need more than one plugged in at a time. The drives are mostly for backup, and (for <= 2TB) like the flexibility of being able to plug them in for higher speeds when needed.

The USB-SATA is USB 2.0 only. I'm _guessing_ that's the issue, but rather check with others here. Before I go an get an USB 3.0-SATA, someone might be able to clue me in on other things to look out for.

Given the PC itself can only support USB 2.0, now I'm curious why the MyBook 3TB even works. There is important data backed up on it.

Any ideas?
Thanks

USB 3.0 is backwards compatible. MyBook will just operate at the lower limits of USB 2.0.

Are you seeing the same size limit on your MyBook as your Green?

No, I only get "746G" with the green, yet My Book gets 3TB, that's the question.

Another mystery, Windows 7 sees the My Book as MBR, not GPT. I think there is a trick used to get 3TB using MBR (larger NTFS block size or something, it was late when I read it)

I'm considering getting a PCIe HBA that supports > 2TB