Error when trying to open dmg files

Hey all, hopefully you can help me out. I'm having an issue opening dmg files. I keep getting an error when I try to open them. It says "device not configured" and the disk image in the dmg will not mount. I'm trying to install a bittorrent program. I've tried downloading two different bt clients (Transmission and Bit Torrent) in both Firefox and Safari. Has anyone run into this problem before or can anyone direct me to a quality bittorrent client? Thanks!

From time to time I too receive error messages when attempting to open dmg files. I generally find the problem is corrected by a re-start, however, if it persists, I use Applejack [the troubleshooting tool] which always rectifies the situation. SourceForge.net: AppleJack:Troubleshooting Tool for Macs

I have no knowledge of Bit Torrent or Transmission.