To erase and format any drive connected to a Mac computer running OS X Yosemite, use Disk Utility. Simply erasing a USB drive in Disk Utility will automatically format it too. Because there are different types of formats for different purposes, it's good to know the differences before accepting the default settings. If you are concerned about privacy, for example, you can format the drive with encryption and a password. Disk Utility can also partition a USB drive, creating two or more virtual drives from the same disk.
Connect the USB flash drive to your Mac. If there are any files on the drive you need to keep, copy them to your Mac computer. Launch Disk Utility, located in the Utilities folder inside the Application folder in Finder.
Select the USB flash drive in Disk Utility's left menu and then click the "Erase" tab.
Click the "Format" menu. If you want to protect the disk with a password, select the "Mac OS Extended (Case-sensitive, Journaled, Encrypted)" format. Otherwise, select the "Mac OS Extended (Case-sensitive, Journaled)" format.
The case-sensitve format can distinguish between lower-case and upper-case letters. For example, "Doc.txt" and "doc.txt," are recognized as two different files. The journaled format protects the integrity of hierarchical file systems.
The ExFAT and MS-DOS (FAT) format can be read by Apple computers and Windows computers. Apple recommends using MS-DOS (FAT) for disks 32GB or smaller and ExFAT for larger disks.
Click the "Security Options" button if you don't want a fast format. A fast format is Disk Utility's default option. It takes only a second or two to erase a 32GB disk, but it's not very secure. Anyone with the right recovery software could still find the data. If data privacy is a concern, drag the slider to the right to use the Most Secure option. This takes much longer to format the disk. The two settings between Fastest and Most Secure balance security with speed.
Click the "Erase" button. If you selected an encrypted format, enter a password and verify your password in the text fields. Each time you access the drive, you will be prompted for that password, although Keychain will offer to remember it for you when using it on your Mac. Type a hint to help yourself remember the password and click "Erase."
Formatting With Partitions
Select the USB flash drive in Disk Utility. Click the "Partition" tab, if you want to use more than one partition in the drive. A drive with two partitions, for example can have one partitioned in ExFAT and another in a Mac OS Extended format, so you can use the same drive on a Mac and a Windows computer.
Select how many partitions you want in the "Partition Layout" menu and specify the format you want for each partition as well as the size and a name. You can have up to 16 partitions in the same drive. Encryption is not an option when you partition a USB flash drive.
Click the "Apply" button. Verify the number of partitions you are creating and click the "Partition" button.
Encryption may not be an available option on older USB flash drives with limited storage.