From what I've been able to gather, that table (mdl_portfolio_tempdata) probably contains no records. It is a valid table not created by backup/restore process. There is also one other in a 2.8: mdl_context_temp. Also a valid table not created by any backup/restore process.
from what I've read about the temp tables created during backups and restore those are true temp/tmp tables ... and when the backup/restore process *session* ends, those tables are removed.
About the only way to assure yourself that is working properly, I think, is to turn on mysql logging on a temporary basis (don't want to leave it on). Make an attempt to restore that troubled backup. After it fails turn off logging. Then go inspect the logs. Those are ascii files and can actually brought up in a text editor (but don't edit and save - just using find feature of the editor to inspect mysql server actions).
I will defer any correction of this response to a true DB admin person - one with certification in said app (MySQL) administration.
'spirit of sharing', Ken