Behaviour of TM toward and extHDD vs TC?

Hamper wrote:

...How will TM behave to an internal HDD when backing up, it will be getting the data from an SSD, for use as my main drive...

It will work just as if it was an external drive except faster because one drive will be talking over the internal bus to the other; no USB 2.0 connection to slow you down. Keep in mind, though, that after the first TM backup, the rest are incremental, and if not much gets changed, not much time is needed for the backup. As to using two internals for boot drive and TM drive, I've been doing that for years with the Mac Pro's multiple drive bays.

But think about how much TM backing up you really need to do. Do you really need hourly backups and daily backups and weekly backups or would you benefit more from running the TM backup when you decide you need one? Turning TM off and using the "Back Up Now" command will allow you to choose the timing for when the Mac is not otherwise busy. This is not a common or generally recommended practice but it's worked well for me, in conjunction with clones.

Because TM isn't enough. It's a really good idea to also make periodic bootable clones of your SSD. Not only do you get a complete slice of time, but when you boot from the clone (something you can't do with TM), you get access to diagnostic and repair software (if you have any) for your system that goes beyond what Disk Utility can do.

BTW, the ultimate resource for Time Machine is here.

A clone of a large drive isn't a snap to backup but I've been surprised to see a full clone of a nearly 700 GB SSD to an external HD over USB 3.0 taking only about 95 minutes, as long as it's going to the first partition on the external. Setting that to run during dinner gets the job done.

At the same time, TM is remarkable in its ease of recovery: a hack on a preference file not working? Just open the directory the file is in, call up TM and recover the old preference file. I agree that if your work involves lots of ongoing changes to lots of things, the automatic TM backup process is ideal. And if it's to the second internal drive, that's always available for the backup and there's no worry about having the external always connected and powered on or space taken up by local TM backups when the external isn't available.

BTW, it was DiskWarrior I was thinking of when mentioning diagnostic and repair software on a bootable clone.

Just a heads-up: here is a discussion of a bug in ML's TM which can corrupt backups. Good thing we can clone, too.

Hamper wrote:

. . .

I had to let it run and because it was tethered by USB I couldn't use the MacBook because the connection was fragile and could be lost, the whole process took me over 24hrs!

The first backup does take a long time (roughly 40-50 GB/hour via USB, overall, slower at first), but thereafter the hourly incremental backups should be small and quick. If they aren't, something is wrong. If so, post back with some specifics and we'll help you investigate.

I did some research and found that TM will not allow a backup to be resumed, although it will pause, when resuming TM just takes forever to carry on from the point it left that you'd be as well to just start again.

Not really. The backup will be cancelled, of course, and when restarted TM will have to do a "deep scan" to figure out what needs to be backed up, but it doesn't copy all the same stuff again -- that's kept in an ".inProgress" package on the TM drive. So it's not recommended, but usually not a major problem, either.

FatMac wrote:

. . .

But think about how much TM backing up you really need to do. Do you really need hourly backups and daily backups and weekly backups or would you benefit more from running the TM backup when you decide you need one?

Sorry to disagree, but in most cases it's best to let TM do its hourly backups. If they're large or slow, something is probably wrong. Those hourly backups will protect you best -- it's kinda like having hourly Versions for all files. If something goes wrong (user error or sudden corruption, etc.), you've got a good chance of recovering an earlier version.

The extra backups usually don't take up any space, because all but the first of the day are deleted after 24 hours; after a month, only one per week is kept.

And, on laptops, space permitting, if Time Machine is ON, it makes Local Snapshots, so if you're away from your external backups, you've got a week's worth of changes. Those are expendable, automatically deleted to keep your HD under 80% full, so don't really take up any disk space. If you turn Time Machine OFF, not only does it quit making them, but all existing ones are deleted.

For your secondary backups, take a look at Time Machine - Frequently Asked Question #27, especially the green box.

Ah, ok, I got the impression even the incrementals were a pain -- shouldn't be.

And, of course, things that would slow down backups to an external will affect an internal too, probably not as noticeable, of course.

Hamper wrote:

This is terrible, all history lost, resetting the TC is not a solution!

Resetting the TC won't erase it.

Pondini wrote:

FatMac wrote:

. . .

But think about how much TM backing up you really need to do. Do you really need hourly backups and daily backups and weekly backups or would you benefit more from running the TM backup when you decide you need one?

Sorry to disagree, but in most cases it's best to let TM do its hourly backups. If they're large or slow, something is probably wrong...

As I'd mentioned, my suggestion isn't a common or recommended practice; for that, the real expert (i.e., you) should be consulted. But my files don't change or are added to all that often; my goal is to avoid having TM kick in at inopportune times. For example, the release 10.8.4 updates have just been posted and I downloaded and archived both of them. Shortly afterward, I started making a clone of my boot drive. That lasted long enough that the automatic hourly TM backup would have started and run concurrently. I'd prefer to have my Mac devote itself to just the cloning rather than running the TM backup too. And the TM process is complex enough I'd just as soon give it breathing room to do its thing. The TM bug I linked to above doesn't seem to have a known cause but I can't help wondering if having TM contend with too many other process might have something to do with it, and that would be difficult to beta-test for.

I know my Mac can walk and chew gum at the same time but with my usage it doesn't have to.

FatMac\>MacPro wrote:

. . .

The TM bug I linked to above doesn't seem to have a known cause

I'm the guy who identified it. See https://discussions.apple.com/thread/5080333 and clarifications I recently posted to the discussion in your link.

It may not really be a Time Machine bug at all; we suspect a rogue app of telling it to exclude system folders. Nothing at all to do with too many processes.

Yes, two large backups running at once may slow your Mac down, as will your internal HD being indexed by Spotlight, so delaying one isn't a bad idea. Better, though, is just go to dinner. 😉