An update on my USB3.0 issue...
I tested my 2 Lacie USB3 drives using a USB 3.0 y-cable with my older Intel Macbook and had the same issue as before (the Mac won't boot from the drives via USB 3.0). So it's not a low-power issue. Looks like an incompatibility issue, likely with BIOS/boot loader on older Macbooks.
I tested a generic IT-CEO USB 3.0 HDD Enclosure with the same older Intel Macbook and it booted up fine. So it's not an incompatibility with USB 3.0, but an issue with Lacie brand drives.
I must say it's very disappointing that a premium brand such as Lacie doesn't have compatible firmware that supports all Intel Macs, while cheap Chinese manufacturers are able to do so.