Help me please wipe my Linux USB or fix it right

I want to apologize in advance for how lengthy this is. I am ruing Ubuntu 14.04 that I installed directly onto a 32GB Toshiba flash drive. So it boots when I tu on my PC with F12. I partitioned it and gave Ubuntu 7GB of space for itself and the rest is FAT storage. Now this is the funky part. I installed it from a trial version off another 4GB USB, so both need to be plugged in for Linux to run. This whole set-up is more hassle then it is worth, so I want to erase my drives and start again later. My issue is that my Toshiba drive (this is the main one I am worried about) does not show up when I boot Windows. I was told that this is because it is now a non-Windows readable file, its now a Linux Hard Drive. So I go into the Boot Loader and open my Toshiba drive to boot Linux and go into Disks to reformat it with zero partitions and all FAT32 storage. So I click either do or do not overwrite files with 0's (same message comes up either way) and Empty with no partitions. I enter my password to Authenticate. When I do I get this message.

Error Mounting Filesystem

Error unmounting /dev/sdc1: Command-line `umount "/dev/sdc1"' exited with non-zero exit status 1: umount: /: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))
(udisks-error-quark, 14)

Screenshot from 2016-05-09 06:23:09.jpg

I started using Ubuntu on Friday and installed it to my USB last night. I didn't know that my extra 24GB NFTS partition would not show up as regular flash storage. So that is my issue. If someone could help me format it so the 24GB shows up as flash storage in Windows, Mac, and Linux that would be awesome. If that is not possible, how do I wipe the drive. Any help would be great!