I have a brand new iMac 27" i7 top of the line model. I bought a USB 3 enclosure from OCW for a 120 gig SSD. I was using it as the cache drive for Photoshop CS6. It began to display the same issues you are describing. I brushed it off and blamed the cheap enclosure I purchased. This all occurred 1 week ago on 1-22-2013.
On Saturday 1-26-2013 I was backing up a sweet sixteen shoot via time machine connected to the USB3 port and the drive ejected mid way of a 70 gig back up. I found that strange because I've never had a problem with that drive or the enclosure it's in. (By the way this is a USB2 enclosure) I tried it 3 more times and the same issue occurred. I figured it was the cable and switched it with another USB cable and this time it worked.
Not thinking much about I assumed the problem was the cable. The next morning I realized I still had one 8 gig compact flash card in my camera and I needed to back that one up. This time I used my USB 3 RAW STEEL UDMA card reader to ingest the work. There were only 20 images on the card. During import the compact flash drive froze on the third image. I had to force quit and redo the import again. It took 3 times before I had no issues importing the images. I could not figure out what the **** was going on until it hit me.
During my trouble shooting I realized the 3 drives were, when they ejected themsleves, connected to the same port in the back of my iMac. The second one closest to the SD card reader port in the back. That was the free port I kept switching to use these different peripherals.
I need to do a little more testing but I'm almost certain something is wrong the the USB 3 port in the back of the new computer. Never had these issues with my late 2009 iMac.
The last thing I want to is haul this machine into Apple after it took me 4 days to get it set up the way I use it for my Photography business.
Disappointed big time! Anyone else have these problems?