Select the drive and click on Get Info. If on Mac you can either get the get info from file menu or right click to get you contextual menu up and select it from there. Have a look what size the drive is and how much space you have on the drive.
More than likely it's got data on the drive already so not enough free space. Even if the drive looks physically empty it could have a load of Hidden files on there that build up over time or in the trash. When you put files on a drive they can also put hidden files on the drive as well. One trashes the file believing that it has all been taken off the drive when in fact there can be stuff left behind you can't see. Also when you trash something off a drive or USB flash memory, that still takes up space on the drive until the trash is emptied. When you eject a drive whatever you had trashed will go along with it when you unplug the drive and you see it in the trash again when you plug the drive back in, until the trash has been emptied. The only other thing I can think off is that the software your using is putting a restriction on how much you can capture.
Best to start with a nice clean fresh drive. The more fee space the better as it will get used as scratch disk space, virtual memory (temporary ram) So either use a new one or re-format a drive your not using. The important thing to do when re-formatting a drive is to Zero all the data so any remains are most definably removed. If on the Mac use Disk Utility which you will find in the Utilities folder. Select the drive on the left (
Make sure to select the right drive. Dont go wiping the startup disk 
), go to the Erase Tab, choose the format you want the drive in (Usually Mac OS Extended) then click on options and select Zero all data.
(:-})
