What sort of Sandisk flash drive do you have? They used to make ones with a built in launcher which allowed you to run portable versions of FireFox, Thunderbird, etc... But on a Windows system only. The Flash drive showed up as two partitions; a CD ROM containing the launcher and portable apps, and the storage area.
If you've one of these getting rid of the CD ROM bit needs a different approach, you can't format a CD rom so it would show as write protected!
If I remember correctly, if the launcher was booted on a Windows systems, (Plugging it in to a running Windows system should cause the Launcher to deploy)you could uncover an option to blow away the System area (the CD bit) and convert it to a storage device only.
After that? Maybe it could be formatted?
Oh! And Welcome to Linux Questions!