Comment 2 for bug 766033

XtraBackup does report the file name when starting to copy the file:

xtrabackup_55 Ver undefined Rev undefined for 5.5.9 osx10.6 (i386)
[01] Copying ./ibdata1
     to /Users/kaa/src/percona/percona-xtrabackup/test/var/backup/2011-04-18_08-49-52/ibdata1
[01] ...done
[01] Copying ./incremental_sample/test.ibd
     to /Users/kaa/src/percona/percona-xtrabackup/test/var/backup/2011-04-18_08-49-52/incremental_sample/test.ibd

So it should be possible to figure out the file where the error has occurred. It's just that the log in the bug report is truncated.