I'm also having this problem. Using a 2TB WD external drive, usb 2, connected to a brand new iMac. The drive was fine for me through Snow Leopard, Mountain Lion, and Mavericks on my old MBP, and for the past month or so was okay on the iMac; now with the latest Yosemite update it has gone insanely sluggish. The drive is about 75% full, so wonder if that is part of the issue, but the problem did pop up rather suddenly. Sometimes, if I wait long enough, the drive will mount, but as the OP complained it will take several minutes to display the contents of any given folder. I'm currently in the process of trying to back-up this data, and Finder tells me it will take 16 hours to transfer a 30gig folder. At this rate i'm looking at a month or more running full-time to save my files, and I'm scared the drive might completely burn up before then. Any guess at what's wrong or how I might more efficiently get the data to a new drive would be appreciated.