Filevault - error message 'an error occurred during copying'

320gb hd - 260gb available. SL 10.6.8

Firtst time use of Filevault and when attempting to turn it on for either Admin User or Standard User and same message is generated. Appears as if it's going through the set-up process, estimates the time it will take and then generates same error message.

Afvice appreciated.