Frequently when I call it a night, I unmount my external USB drive, turn it off and put my MBP to sleep. Frequently I find that my USB hard drive when turned back on does not mount and sometimes I can see it in Disk Utility and sometimes not. When I can see it if I select the drive and hit the "mount" button, nothing happens. Of note this drive has 2 partitions. Is this a known issue? What is the solution? Thanks!