I found a solution on macrumors:
To successfully prepare a disk for Mac that has been previously formatted for a PC (as most new drives are of course), the disk needs to have the "Partition Table Map" established for Macs rather than leaving it as is.
• Start up Disk Utility
• Click the disk to be formatted in the left column. Click the whole drive line, not the partition(s) if any show below it
• Click the "Partition" tab at the top of the window
• Change the "Volume Scheme:" from "Current" to "1 partition" (or however many partitions you want)
• Click the "Options" button. Choose the appropriate partition table type in the window that appears- GUID for Intel based Macs, APM for PowerPC based Macs). Click OK to close that window
• Click on the partition (or partitions one at a time) and enter a name in the "Volume Information:" section and choose "Mac OS Extended (Journaled) in the "Format:" section
• When done naming, click "Apply", then "OK"
• Sit back for a short time and when complete enjoy the drive!