I've the same problem.  "export_dir": "/storage/updatemgr/migration" setting on .json file didn't work. I created "migration" folder before the upgrade.

The log shows:

Running precheck: RecommendOrCheckExportDirTask execution failed

Error message: Not enough disk space on '/' for exporting source data 19.34 GB

including migrate set 'all' in export_dir '/storage/updatemgr/migration'.

Any idea how to point exporting source data to "/storage/updatemgr/migration" ?

Jouni