I have this strange problem with my 3Tb MB Essential. The speed of the disk comes to a crawl when I am at 170GB or less in free space. Copying a 1GB file to it takes hours. If I free up space, it works, until it’s down to 170GB or less.
I have checked the disk in Disk Utility and Smartware and it desn’t find anything wrong with it. I have updated it with the latest FW etc.
The disk is GUID partitioned and 1 large partition formatted as journaled and case sensitive.