Backup to external USB hard disk

Question

I've been trying to use rsync to backup my internal HD to an external USB one. I'm attempting to backup everything from /Users/ on down. There are two users on this G5, and attempts to access the other user's directories/files are being blocked with "permission denied" message.
Hoping root could reign with access privileges to all users, I've used "sudo -u root crontab -e" to set up a crontab entry to invoke a file containing the actual backup command itself:
"rsync -av --exclude-from=/Volumes/'Macintosh HD'/Users/jdorin/SysBackupExclude --progress /Volumes/'Macintosh HD'/Users /Volumes/MacBack > echo -n"
The SysBackupExclude file contains just:
/Macintosh HD/Users/user1/Library/Caches
/Macintosh HD/Users/user1/.Trash/Caches
/Macintosh HD/Users/user2/Library/Caches
/Macintosh HD/Users/user2/.Trash/Caches
I'm "user1," and my files are getting backed up. But the emailed backup status report tells me "permission denied" for each directory under "user2," as:
rsync: opendir "/Volumes/Macintosh HD/Users/user2/.Trash" failed: Permission denied (13)
rsync: opendir "/Volumes/Macintosh HD/Users/user2/Desktop" failed: Permission denied (13)
How can I backup all users on my Mac?

Answer 1

I never unplug the external harddrive, and the error message comes up only occassionaly upon startup.  Maybe the USB connection gets disrupted enough to cause the computer to see it as disconnected and then reconnected while the iMac is powered.