Hello Martin,

MG> Similarly, I use two USB 2.5" hard drives (currently a pair of 375GB WD
MG> Blue hard drives) to take weekly backups. This is mild paranoia: I use
MG> two, rotated in sequence, so one is always offline in a firesafe and so
MG> fairly safe from damage.

I never want WD drives ;-(.
I have to many friends who got problems wit that make and types.

MG> However, IMO the key trick is using rsync to do the backups. Its fast
MG> because it always does the minimum work needed to make a full backup: it
MG> only copies files and directories that have changed to the backup disk
MG> and removes files from the backup that have been deleted from the disk
MG> being backed up. The firdst backup to a hew disk is always slow because
MG> everything is backed up: subsequent backups can be 20-30 times faster
MG> depending on how much has changed/

Yes that's ideal, it is called mirrorring.
The destination drive is after the selective copy a complete imaga of
the source media.

But what happens when at the destination there is a file called ProjectX,
and on the source media it became a directory called ProjectX, with several
files more in them.
Is then de original file ProjectX deleted before making a directory ProjectX at
the destination, or does rsync give an error that the directory ProjectX cannot
be created, as there is already a file with that name?
Then te backup copy process is interrupted for an answer to gon on.

MG> There is a related program, rsnapshot, that preserves deleted files, but
MG> I haven't used it. I think it works like this: like rsync, it initially
MG> makes a full backup as the base snapshot and then preserves a set of
MG> snapshots, with the files and directories in each being either a copy of
MG> a file that changed or a symlink the most recent backed-up copy.

Did you test this?

MG> Either of these is probably faster than using dd for backups and is
MG> certainly much faster than using an archiver such as tar (with or
MG> without compression), zip, lharc, etc.

Yes of course, but is it working secure?, and has that been extremely tested?
Because everyone wants to be sure to have good, complete and accurate backups.

I once bought and paid for a commercial program called !SaveStore for RISC OS,
but it could NOT do secure mirror backups. I.e. to delete the files at the
destination that were removed from the source media,
and copy only the missing objects.
It did the copy partially, and not always the deleting.
I tested that many times and it was very unreliable ;-(.
I tried wit with a simple set of subdirs at a RAMdisc.

!DirSync is a much more reliable directory and file copying program for this
kind of work. But there something goes wrong either.
The problem lies in files that are the same, but treated as different,
or just the other way roud. I.e. the name, date and length are the same,
but in that case you have to do a binary test for equality. I.e. they still can
be different. Same name, date and length are the same, but the contenst could
nog be 100 % identical. Then you have to make a decission what to do?

And when files are 100 % identical, the oldest version should be kept, as
that is likely to be the original, and the other one "stamped".
Many programmers always stamps even unchanged files, arg... ;-(.
Please donot do that.
So now I am mostly copying manually, but that takes more time ;-(.
I am using a RISC OS tool very much to stamp the original file to its original
date ansd time stanp, if there are more incarnations of the 100 % idential
contents and name of that file(s).

Now I am busy to backup all my USB sticks to an external USB2 HDD with an
old Win2K laptop I once did in 2017, and I have to think what could be faster,
delete a complete subdirectory, or copy all over it and delete the removed
items.
If you do not do the latter, the backup is always increasing bigger and bigger.
So I like the mirror trick, but there is not much backup software that does
that good on many platforms, i.e. Windows, RISC OS, and Linux.
So, if you (or someone else) have suggestions, I am all ears
to receive your comment with the only half one ear I have ;-).

Henri.

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