My Book Studio 3TB USB 3.0 drive not Bootable

WD External Drives External Drives for Mac
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I have a similar issue. When trying to boot up my oldish iMac 2.4 GHz Intel core 2 Duo, 4GB 667 MHz DDR2 SDRAM, the desktop will not boot up and gets stuck on a grey screen. If I disconnect the WD drive the computer will boot up. I then have to plug in the WD drive again and the drive will appear on the desktop. It does appear to back up on an hourly basis but I’m worried plugging and unpluuging the WD drive will cause damage. It’s scertainly not a very practical or smooth way of using a backup drive