I have a brother who is paranoid. He is starting to use an external hard drive to back up his information on his Mac. He says he is afraid that this might erase information from his computer hard drive. What can I tell him to allay his fears?

Say, "It will not erase the data on your computer." After which you should encourage him to take his medication. 🙂

A file cannot disappear from one physical drive or partition when performing a default copy to another drive/partition.

When you drag and drop files or folders from one drive to another, the mouse will get a green circle with a white + symbol next to it. That means it's a copy. In order for it to be a move (where the files will be removed from their original location after each copy is complete) you have to hold down a modifier key. In which case, the mouse will have no symbol next to it.

Non technical explanation, green + symbol is good.

Just tell him to do it and check to see if it loses data. No theoretical explanation will cut any ice.

I'm afraid there's little then we could say that would ease his mind. Some people are just downright afraid of computers and won't use them unless they have to.

Making a backup will NOT affect the original data.

In fact failure to backup his data is more likely to result in data lost sometime in the future.

Disk drives are mechanical devices, and subject to failure.

If he is really paranoid, then he should be using the 3-2-1 backup method.

3 copies of his data including the original

2 different backup formats as one format is a single point of failure.

1 copy should be off-site.

If he uses an on-line networking backup solution such as CrashPlan, Mozy, Carbonite, Arq, etc... that would satisfy both off-site as well as using a 2nd backup format from Time Machine. The off-site protects him from theft and natural disaster. If not going to use an on-line networked backup, then consider using a cloning utility such as SuperDuper or Carbon Copy Cloner to an external disk and then take the disk to a family member, friends, or work. Several disks for cloning means you can cycle through the disks always keeping at least one off-site at all times.

Also a backup is protection against his own mistakes in accidentially deleting or modifying a file he really needs.

If he is really paranoid, he should be backing up Big-Time, not thinking of reasons to avoid backup. That is just asking for trouble.

Not trusting the software to do what you told it to do is a type of fear. "Afraid" doesn't necessarily mean you can't approach an item without trembling. His fear, if I've read your posts right, is the loss of data.

BobHarris has outlined a well known redundant backup plan.