I've got a 32gb usb with 790mb used for the ubuntu boot so about 31gb free space. How can I format this free space to fat/ntfs so I can use it for other things? Thanks!
Probably the easiest way would be to format the whole drive, hence also deleting the current Ubuntu partition on the USB.
You do that by using Gparted, an app which are available for free download through the Ubuntu Software Center.
When open in Gparted, you choose the USB-drive in the upper right corner of the app.
Then you right-click on the current partition and choose erase (has to be unmounted first), then click "Apply".
After that the whole USB drive will be seen as "Unallocated Space".
Now right-click on the Unallocated space and choose "Create new Partition" (or similar), and choose the new partition size (the one for Ubuntu has to be around 1 GB and fat32) Repeat the process to create all the partitions you want.
Now use the USB-startdisk-creator which comes with the default Ubuntu installation (seach USB in the dash). Select to install Ubuntu on only the partition you created for it.
Now you will have a partition for the Ubuntu-bootable and one that is empty
If you were only using 790 MB for Ubuntu, the system was probably already FAT32. A Full install takes over 4GB. Did you try saving data to it as is? Most of my USB drives have a Persistent install of Ubuntu on them and I have no problem using them for normal things. To access the data while booted on the device go to filesystem/cdrom as root.
You can also do a Full install to the drive using manual partitioning and make the first partition FAT32 and the second partition / using ext4.