CQ5 online backup failed since using shared network drive as target dir

I am testing CQ5 online backup with both UI and Curl. With local drive as target dir, it works well.

curl -u admin:admin -F "target=c:/cq5backup/author/author_backup_%date:~4,2%%date:~7,2%_%tim e:~0,2%%time:~3,2%.zip" -F "force=true" -F "delay=1" "http://localhost:4502/libs/granite/backup/content/admin/backups.html"

However, if I use shared network drive in the target dir (e.g. h:), it failed with message in error.log "com.day.crx.core.backup.Backup Failed to create temporary directory ".

curl -u admin:admin -F "target=h:/cq5backup/author/author_backup_%date:~4,2%%date:~7,2%_%tim e:~0,2%%time:~3,2%.zip" -F "force=true" -F "delay=1" "http://localhost:4502/libs/granite/backup/content/admin/backups.html"

BTW, I can copy any files to drive h: without permission problems.

My environment is Windows 7, and CQ5 is started as a service.