I have a relatively new iMac and recently purchased this product. It never worked. I went through extensive reformatting exercises requested by LaCie's tech support. It took about a month. I purchased the product through the Apple Web Store. After a month Apple would not deal with the product and required that I go back to LaCie. LaCie was okay with replacement but gave me two options: (1) send the product back and they try to fix it and return it, or (2) purchase for $15 an "advanced return" where they send a new one to me and I return the old one in the box. I chose the latter because I NEED the hard drive now! However I am neither happy with Apple nor LaCie about this whole thing including having to pay an extra $15 for a product that Apple advertises but apparently is defective.
This is the second time in 6 months that I've purchased what appears to be defective peripherals off of the Apple web site. The first seem to solve itself when I moved from Lion to Mountain Lion. That can't be the engineering of the peripheral, it has to be Apple's inability to insure plug and play, and therefore it is a problem of the iMac. I'll have to wait to see whether the "new" LaCie drive works. If not I'll be purchasing a new computer, and it won't be an Apple product.