Creating a bootable clone

So first, CAN I AT ALL USE A FLASH DRIVE AS A BOOTABLE DISK?

Yes, but flash drives are incredibly slow compared to any hard drive.

If not, how can I use my ext. drive for that? It's 1TB size, and already divided to 3 partitioned. MY QUESTIONS ARE:

--- Can I use DiskUtility to CHANGE THE SIZE OF AN EXISTING PARTITION, MAKE IT SMALLER? (There's no need for more than about 20GB for a startup disk)

--- CAN I ERASE/REFORMAT ONE PARTITION, vs. the whole drive?

Yes, to all of the above. Easiest thing to do is use Disk Utility to remove/change the size of the existing partitions. You can also of course erase and repartition, especially if there's nothing on it right now, or only a small amount of data you can temporarily copy to they main drive, and then copy back.

Disk Utility can clone a drive. You can also use Carbon Copy Cloner or SuperDuper!.

As to the Flash drive, does it make sense for it to take an HOUR to show the desktop? And what could be the reason my int. HD did not mount?

An hour is excessive. I couldn't guess why it would take that long. The hard drive most definitely should have shown up on the desktop. When booted to the flash drive, check the desktop prefs and make sure the option to show drives are checked.

As to the ext. drive, so you're saying I can play with the partitions' size and reformat one single partition, right?

It may be a bit more complicated than that, depending on how you want the partitions to be resized. But again, as long as there's currently nothing on the drive, are a small enough amount to temporarily place somewhere else, it would be easier to repartition it with Disk Utility by creating all new partitions.

I still prefer to use the Recovery Partition of my new iMac to create the startup disk (partition)....

Either would work for recovery, but the point of cloning a drive is the ability to clone it back; third party software and all in the last state you last backed up the drive. Then you don't have to install the OS, then your software, then your data, or doing a Time Machine linked restore while installing the OS. Much, much faster to just clone a recent backup back to the main drive.

Also, you can't run any third party apps from a Recovery Command+R boot. You can do the minimal choices available from that screen. In order to use DiskWarrior or other app, you must boot to another fully capable drive.

As long as what's on the current TM backup isn't important, yes, you can just erase it. Your main boot drive will simply ask you to designate a new drive. Which you can do after repartitioning.

The Retrospect data can easily be dumped into a folder on the main drive as a place to save it while doing the repartitioning. Just copy it back when done. Retrospect may need the new partition to have the same name as it is now in order to locate the data when you copy it back.

Then I can reformat the ext. drive, repartition it to 2 partitions, and as you suggest, clone my int. HD in one partition, and copy back the back-up files, and dedicate the 2nd partition for back up only (both my old, and new incremental Retrospect and Time Machine).

I'm not sure about the second part since I never use TM. I don't know how happy it would be about having the Retrospect data on the same partition. You'll want to wait and see of others have advice on that.

Question: Can I later change the proportion between the 2 partitions, namely reduce one, so I can enlarge the other (as long as there is free space in the one I want to reduce)?

Yes. You open Disk Utility and drag the lower corner of either current partition up to make a blank space on the drive. Then click the + button to fill the space with a new partition. The partition you're stealing space from of course can't be so full that you can't modify it.

Wait, but that would create a THIRD partition, no? I simply want to have the option to later give partition 1 more space -- on the account of partition 2.

Oh! Sorry. It can be done, but it does require removing the second partition and redoing it. Like this example.

Here I moved the grab corner at the lower right of the upper partition to make a blank area. I an now press the + button to create a third partition. What you can't do in such a case is grab the top of the lower partition and move it up to fill the space. It's initial position on the drive is locked. So what you'd have to do is backup whatever data is on the lower partition, delete it in Disk Utility (in the work space shown above), shrink the upper partition and then press the + button to fill the now fully blank area below it with a new second partition. Then restore your data.

Can't say I've ever seen that one before. How did you go about trying to redo the partitions? It has to be done by launching Disk Utility and choosing the physical drive you want to modify, such as below.

Did you remove or resize a partition with Disk Utility and then try to erase the remaining second partition with the unused area still blank, or not updated? If so, that would cause an error. You must complete any partition changes you want and click Apply at the lower right of DU so the drive's new setup is complete. Then you can erase the new partitions. Though as long you set the new partition(s) as Mac OS Extended (the default), it's unnecessary since they're already blank.

The Mac, for whatever reason was claiming that some program is running from that ext. drive, and I couldn't even manually drag it's icon to the trash to unmount it

Oh, that's why. Even with no apps open, OS X will occasionally believe something is still using a drive. The correct way to release it is to restart.

BUT, I couldn't use DiskUt to "restore" the OS, it said I must restart from the Recovery Partition (restart with Command-R), run DiskUt from there and do the job... I think this is a new, Lion thing...

No, you should have been able to reformat and repartition the external from the main drive. That likely came back to force dismounting it. The OS still believed something was using files on the drive once it was mounted again, so still refused to allow you to use Disk Utility on it.

Ah, well. You've got it done the way you want now.

That is just bizarre. I actually can't explain either instance. Restarting should always release any open files since a restart to the computer is the same as starting cold. Nothing has been opened yet.

The only thing I can think of is you may have had a startup item that pointed to an file on the external drive.