I try to create a startup disk with Ubuntu 15.04 64bit on a 16G USB stick. When I start the "Startup Disk Creator", it will show progress of "Copying files..." until about 49% complete, then hangs forever. No error message shown, no hint if anything is still tried or being done or if I am looking at a corpse. When I press the "Cancel" button I get a confirmation dialog showing "Do you want to quit the installation now?" but when I click "Quit" there, now that dialog also hangs. After killing the process, when I retried but disabling the extra data region of 1G that it creates by default, the process indicator went to 96% complete and then kept being stuck there.
Startup Disk Creator is known to having issues. Use the Disks tool to create the Ubuntu USB installation medium. Open Disks and select the USB drive (on the left) to be used as medium.
Then select Restore Disk Image from the menu on the top right of the application. Choose the
Ubuntu installation ISO file - check if the USB drive to write it to is correct and Start restoring.
I think this is related to a general problem of copying large files to USB (reported here: copying files to usb stick never finished).
I just waited for ten minutes and it copied over OK. I don't know if cp is actually being unusually slow, or if the speed is just reported incorrectly.
Startup Disk Creator
The Startup Disk Creator in versions before Ubuntu 16.04 LTS was affected by several bugs. It was an extracting tool. But in 16.04 LTS, the Startup Disk Creator was converted to a cloning tool, which is very reliable.
Use Disks alias
gnome-disks to create the Ubuntu installation media in Ubuntu 14.04 LTS and newer versions.
Restore Disk Image from the menu on the top right of the application will clone from the iso file to the target device (the USB stick).
Install and use mkusb in Ubuntu 12.04 LTS and newer versions and other main linux distros.
See this link with more links and details.