How can I format a USB "thumb drive" so it will be readable on OS X and Windows?

"You want to format the drive using Fat32, which is most readable. I'm not sure about the Mac procedure, but to format the drive in Windows you right click on it in (My) Computer and select ""Format..."" Set the file system to Fat32 (it should be the default, but you should make sure). Also make sure that ""Quick Format"" is UNselected, as it does not erase the data. Then click OK and it will start formatting the drive. It will warn you, but I'm going to reiterate (preiterate?) DO NOT REMOVE THE DRIVE AT THIS TIME. Also be aware that this will remove all the files, though not necessarily securely.

Also, depending on the sensivitity of the information and the people you are afraid might read it, you might want to do further erasing procedures. I know that the Mac allows you to delete files securely. You need to do this BEFORE reformatting the drive, or there will be no files to erase securely.

EDIT (years later): You might also try formatting the flash drive UDF. This is harder than formatting it FAT32, but can store large files and has some other nice features. Since UDF is a file system that optical discs (CD, DVD, etc.) often use, it is understood by all major OSs."

"On leopard, you can use DiskUtility to format a volume as fat32.

Insert the usb drive
Launch Disk Utility from
/Applications/Utilities/
Select the drive on the left pane
On the right pane, click on the
Erase tab
Choose Volume Fomat as MS-DOS(FAT)
Click Erase

If you are storing particularly sensitive data on the drive, you may want to perform a ""Secure Erase"" instead -- to do this, after step 5 above, do:

Click [Security Options...]
Move the slider one spot to the right (this is sufficient for all modern drives)
Click [OK]

Now proceed with Step 6 above. This fills the disk with zeroes before performing the format; it takes a while, but ensures that data is actually erased rather than just marked as erased.

Note that with modern flash media, even this step is not entirely reliable and data recovery may be possible. You have to weigh the trade-offs you're willing to make."