First time backup note added about security descriptors
$DirErrors analysis removed because Get-ChildItem is not used anymore in main loop.
Make-HashTableFromXML function reworked

It now prints the hashfile it loads. Also, for each non-existing hash
value, the path to the file is checked. If the path points to a file,
hashtable is updated. This should remove the error where hash points to
uniexistant files, but it is a bit slower.
Some comments deleted
Comment adjusted
Minor code cleanup
shadow of backup drive is also made in case something goes wrong