If you’re running both Mac OS X and Windows 7, you might have encountered the problem of your USB key(s) either working on the Mac OS X or the Windows 7, and not on both of them.

Following the below guide will fix the problem:

Step 1) Partition the USB key using Disk Utility on Mac OS X. Make a single partition using the format “free space” and the partition scheme “Master Boot Record” (under Options).

Step 2) Insert the USB key in the Windows 7 machine. This will result in Windows 7 telling you that the key needs to be formatted. Ignore this message for now, and access Control Panel -> System and Security -> Create and format hard disk partitions (under Administrative Tools). Now, the Disk Management window should show up.

Step 3) You should be able to see the USB key here, possibly with the text being lined through. Right-click on the USB key tab and choose “New Simple Volume…”. Then, choose to use the entire disk, choose FAT32, choose to use the standard allocation size and then, choose an appropriate name for the USB key.

Now, all you need to do is finish the actual formatting and the USB key should work both on Mac OS X and Windows 7.