I've just noticed this strange behavior, new to 1.9 and 2.0, whereas functionality worked OK in previous Moodle versions.
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PROBLEM
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Checked with 2.0, but same problem exists in 1.9 too.
Logged in as Admin, make a partial backup of any course (other than site front page), e.g. Course 101. Save backup file locally.
Go to Course 102 and go to Restore. Upload previously saved backup file to Files -> backupdata. Click on Restore.
Message: Later in this process you will have a choice of adding this backup to an existing course or creating a completely new course.
Click Continue.
# Creating temporary structures
# Deleting old data
# Copying zip file
etc.
Click Continue.
Restore to -> Select *Existing course, adding data to it*
On the next page, as per moodle 1.8, you should be offered the option to ***Choose a course*** from the list of existing courses on your moodle site... but you are not presented with this option. You only see the Restore this course now button. Click on it, the restore process seems to work:
# Using existing course
    * From: Questionnaire module Demo (QUEST_DEMO)
    * To: moodle20 (moodle20)
    * Adding data to existing
# Creating groups.
etc.

but the restored data can be found nowhere on the site!!!
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DIAGNOSTIC
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The problem arises from file backup/restore_check.html lines 247 & sqq. where we strangely have a commented out line, and conditions which are never met for the display of the "choosecourse" message...

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SOLUTION
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I do not understand enough of the workings of the restore functions to suggest a solution but I do hope one will be found, and I am really surprised that this bug has gone un-noticed (in 1.9) so far..

Joseph