I have a 4TB Seagate Desktop HDD, which I have connected to my mid 2009 Macbook Pro via an old Seagate FreeAgent GoFlex dock (http://www.amazon.com/Seagate-FreeA...&qid=1379982411&sr=1-10&keywords=seagate+dock), using FW 800. It has started to behave sporadically lately though -- it will work fine for about 15-20 minutes, then it will essentially freeze up. No files can be copied, and it won't be ejected. However, if I just pull the plug, and plug it back in, its fine again (for awhile). I wasn't sure if this was because of a hardware or software issue (corrupted file, perhaps, causing some sort of read/write error?), but I didn't want to take any chances -- some of the information is not backed up elsewhere. So I went ahead and bought an identical drive, along with a JacobsParts USB3.0 SATA dock to connect it to my macbook pro. (http://www.amazon.com/gp/product/B00A2WMPOA/ref=oh_details_o03_s00_i00?ie=UTF8&psc=1) After babysitting it the past few days (it had a couple of "freezing" episodes), I managed to copy all of my data over new the new drive. However, I seem to have a problem. When I swapped HDDs (from the Seagate Dock to the JacobsParts dock, and vice versa), the drives won't show up on my system. I get the "The drive you inserted is not readable by this computer" message. But when I switch them back, OS X recognizes them just fine. Any clue why would this be? I have uploaded some screenshots. I noticed that when the drives are not recognized, the partition scheme is shown as MBR. Why would this be, when they are formatted as GUID?