Easiest thing is to trash the TM plist and see if that helps.
Does the message occur at startup of TM or somewhere along the way? Does it say anything beside "unspecified error"?
Is that 190 GB the space that is still empty? How big is the amount of data you are backing up?
Make sure you don't have any mounted dmgs that may add to the size.
It may have a corrupted file that is stopping the copy.
Do this:
Turn off TM
Go to your backup volume, backups.backupd, "your computer name", and then select and trash "In Progress" or "Latest" (it will be the last one in your backup folder listing)
Turn Time Machine back on and either wait for the next backup cycle or force an immediate backup (control-click on TimeMachine, select "backup now")
If this doesn't help open up Console application. Click on the Show Log List button on the left side, then select the All Messages item under "Log Database Queries".
Now enter "backupd" in the filter text area in the top right to see all of the Time Machine related messages. That should give you an idea of what is actually happening with Time Machine and give you a clue as to why it may be giving you an error.
Here is an application that may help monitor and fix TM issues:
http://www.carnationsoftware.com/TM_Error_Log_WebPage.html
