How to fix a USB that won't reformat or delete files

The drive is not unmounting. Try this instead:

1. Restart the computer.
2. Immediately after the chime press and hold down the "COMMAND" and "R" keys together.
3. Release the keys when the Utilities Menu appears.
4. Select Disk Utility from the Utility Menu and click on the Continue button.
5. Select the USB device from listing. Click Unmount from the toolbar icons. If the device ghosts out in the listing:
6. Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.
7. In the drop down panel set the partition scheme to MBR. Set the Format type to MSDOS.
8. Click on the Apply button and click on the Done button when it is activated.
9. Quit Disk Utility.

If the disk will not unmount then quit Disk Utility. From the Utilities' menu select Terminal. At the Terminal prompt enter:

diskutil list

Locate the entry for the flash drive and note the Disk identifier number and the Slice number on the right. It may, for example, be /dev/disk1s2. Yours may differ. Now, enter the following at the Terminal prompt:

diskutil unmountDisk force /dev/disk1

If the command succeeds then quit the Terminal and relaunch Disk Utility from the Utility Menu. Try partitioning and formatting again.

Drive Partition and Format

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the USB drive (out-dented entry with the mfgr.'s ID and size) from the side list.
3. Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to MBR. Set the Format type to MSDOS (FAT32.)
5. Click on the Apply button and click on the Done button when it is activated.
6. Quit Disk Utility.

Test if you can read/write the device. If not then the disk was probably formatted NTFS. OS X cannot write to an NTFS formattted device. Take it to your Windows machine and reformat the disk using FAT32. You should now be able to use it on both platforms.

I'm not up to date on MSDOS Fat formatting but you may be running up against a volume size limitation. Try formatting the drive ExFat which both Windows and Mac can read/write to.