How to make TM start a NEW backup

Question

Since January and updating to 10.5.6 (not sure if that had anything to do with it), my iMac had been freezing up and required me to manually power off and on, at first a few times a week, later increasing to several times a day. I suspected a HD failure, but no hardware tests or SMART status tests indicated so. Since February, Time Machine has been doing backup. A couple of weeks ago I finally had time to do a reformat of the iMac HD and did a full reinstall and updated directly to 10.5.7. Since then I have had no freezing issues.
But. I now have the TM backup from February till mid-May on my external HD. I would like to keep it as it is, all 200Gb worth, and have TM start a totally fresh backup in a new folder.
After reading pages of information about TM, it seemed people have had trouble doing the opposite, i.e. make TM use build onto the backup which was already there. So I figured TM would do what I hoped; make a new folder in the Backups.backupdb folder with the new computer name (I changed the name on purpose when reinstalling).
I set up TM preferences to use the same disk as before, as there is still more than enough space. I was surprised to see what happened:
- The backup folder name changed from the old "EX iMac" to the new installation's name "Exxx Xxxx's iMac".
- A new dated folder (....inProgress) was created, but with the same date as the last backup made from the old installation a couple of weeks ago, rather than today's date.
I cancelled the backup right awayas this wasn't what I was expecting, so the "folder" name is still "2009-05-16-124351.inProgress"
- When I enter TM through the UI, the HD name has been changed for all the old backups (used to be "Macintosh HD", is now "Mac HD ny").
So it seems that TM in its wisdom recognizes the iMac as its old self, and wants to continue building onto the old backups. This is exactly what I don't want... when space runs out on the external HD, I don't want the old backups to be pruned at all. I want to keep the old backup as-is, for future reference etc., and have TM start a full, fresh backup of my near-empty iMac, which it can do its incremental and space-saving thing with later on.
Any ideas as to how I can persuade my computer into agreeing that this is a worthwhile concept?

Answer 1

Pondini wrote:
Just why do you want to prevent TM from deleting old backups? If it's because you've been deleting things from your internal HD, trusting TM to keep it's copies indefinitely, that's not a good idea.
The reason is, TM's copies are no longer backups: now they're your originals. When (not if) your TM drive fails, you may lose all of them.
Thank you for your input. As I wrote earlier, I have reformatted the internal HD and reinstalled a "blank" system. So yes, most files, documents etc. are now found only in the TM backup. One reason for not simply doing a full restore from the TM backup was to allow time to make sure the internal HD was not in fact failing, and being able to reintroduce things step by step in case the problem should materialize again.
The whole point of wanting to keep the existing TM backup as-is is to make it be there "indefinitely". That doesn't mean I trust the external HD to keep it safe for all eternity, but as long as I am trusting it to back up my stuff, why not let it keep my old stuff as well? Although, admittedly, it is space consuming and the important stuff will be present in both the old and the new backup, once I get it running. Old backup > iMac > New backup. You should see how many papers I have in my shelves from years back