I was wondering the same exact thing! Looking for an answer!
Sorry for the delay, shipping took a bit and I have been gone and laptopless.
Basically, I got this card (Brand GMYLE) on Amazon for $10 total (
http://www.amazon.com/gp/product/B0...&pf_rd_t=101&pf_rd_p=470938631&pf_rd_i=507846)
Obviously it probably wouldn't work on mac... but i tried. Once I got the card I tried installing some drivers (caldigit, LaCie, etc), and none worked. On a whim tried Oyendigital drivers (their card is similar, and I based off of a comment on amazon that it worked with some issues)
Tried my 1 USB3 drive on USB2 using blackmagic disk speed test (MAS), got around ~30mB/s Read/Write
Tried it in expresscard and got ~40mB/s read and write in the beginning, but it was jumping around to ~100+ mB/s read and WRITE. True indeed, my drive is faster- I am very happy that it works ATM
Pros:
Supports USB3 AND USB2 devices
FAST- ~100 mb/s WRITE
Hot Swappable (Works perfectly, BTW)
Flush with laptop
Cons:
Gets a bit warm in slot, but ok by me
Ports are a bit close, and card is a bit easily pulled out by pulling device plug.
Does NOT support USB3 power specs (gets ~600ma total, rather than ~900ma per port)
Cannot support 2 powered (eg external portable hard drive) devices at once, but it can do 1 at a time (< havn't tested this yet.., but highly probable)
Due to bus speed might not hit full USB3 speeds, but pretty close. Need to find a USB3 SSD drive to truly test speed.
So YES, even without 10.8 ML, you can get USB3 on your mac (I am even happy just to get 2 more USB2 ports
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Let us know how it goes. I have an express slot on mine too. Haven't used it yet.
See above comment.