Although the disk was encrypted, the GTC is currently in the process of moving over to an electronic data transfer system, it said.
"Because we recognise that no encryption system can ever be entirely infallible, we have taken urgent steps to put additional security measures in place for the affected records," said GTC registrar Alan Meyrick, in a statement.
"We have written to teachers affected to apologise and to assure them that steps have been taken to prevent records being accessed or misused in any way."