Can I backup a full linux system from a ext file system to a external ntfs partitioned disk using rsync -a (keeping all file permissions)?
There have been improvements in preserving POSIX file metadata through the NTFS drivers, but there are many issues in preserving a full system: ownerships, permissions, timestamps, and hard links. It is unlikely to be perfect.
You could use backup software such as clonezilla
(or an archive format such as tar), but these make incremental backups difficult.
One solution would be to loop mount an ext file system on the NTFS drive, and use that for your rsync
backup. The backup would be inaccessible to the Windows system (since the virtual file system would be inside a single Windows file), but that might not matter for your purpose.
And can I backup files like photos from ext to ntfs without loosing any meta data?
The metadata associated with photos is not a problem, because it is stored inside the file, not externally in the file system. Likewise for the metadata of other media (music, video, documents, etc).