Time Machine does (more or less) do backups incrementally, but what you end up with is really more like a whole series of full machine backups (though they aren't ones that you can boot from, the way you can with SuperDuper's disk clones).
And I have had occasion to restore my whole machine from a Time Machine backup, and was happy with the results (it restored all of my folders and applications and all of Mac OS X just fine; it left off some log files that are mostly of interest to Unix nerds, that I would have liked to have, but that wasn't a major problem).
(By the way, rebooting a machine on a SuperDuper backup is reassuring, and a nice sanity check that things are probably mostly okay, but it is absolutely
not a thorough test of a backup -- unless you then proceed to carefully examine every single file on the system -- it's more like turning your key and having your car's engine start, and then thinking that, therefore, your car is in perfect condition -- of course many things could still be wrong.)
(And FWIW, I'm quite interested in the field of backup technology, was in charge of the backups at a company I used to work for, and engineered my own backup systems for their use, and we used those systems on several occasions to save the company's butt.)
I run Time Machine and SuperDuper on the same drive with no problems at all, and I don't believe there's any meaningful risk in doing this. SuperDuper was designed to handle precisely this situation, and Time Machine doesn't venture outside of the Backups.backupdb folder in the root directory of the hard drive, so they don't step on each other's toes. My work machine has its (500GB FW800) backup drive attached all the time, so it does Time Machine backups every hour, and I refresh the one SuperDuper clone on it every few weeks (and I taught SuperDuper to ignore a number of large directories that I know Time Machine is handling, like my iTunes collection, to keep the size of the SuperDuper clone down to leave more space for Time Machine).
Time Machine does delete backups as space on the backup drive runs low, but it uses a fairly smart algorithm (quoting
Apple): "Time Machine saves the hourly backups for the past 24 hours, daily backups for the past month, and weekly backups for everything older than a month." And when it deletes a given backup, the only files you can "lose" are those that weren't around before that backup
and weren't around after that backup.
Time Machine has several huge wins over most other backup methods...
First is, it's so d*mn easy to use that you actually will use it. When a drive fails, the Time Machine backup that you actually did make, and keep up to date, just by leaving the backup drive plugged in and turned on, is
way better than any other backup that you had been meaning to do (or meaning to run again), but hadn't gotten around to doing. Good intentions don't count for anything when you need to reload the drive.
Second win is, it tracks changes to everything on the hard drive (minus, as noted above, some things like a handful of log files that most people will never notice). This is a huge win over any backup plan that involves, "I back up the folders where my work is," because if you put a file somewhere else, you might fail to add that file to your backup strategy, but Time Machine will always catch it, since it looks everywhere. "Oh yeah, I guess I forgot to add
that folder to my backup" also doesn't count for anything when you need to reload the drive.
SuperDuper is much faster if you have to reload your entire drive, but then, on the rare occasion that I lose an entire drive, I don't care whether it takes one hour or four to recover, I'm just extremely thankful that I have backups and can recover the drive at all. If you're losing drives at a rate where that 1-hour-vs-4-hours deal really starts eating into your work week, perhaps you should be taking a hard look at why your drives are failing so often (a good first shot would be, how clean is the power you're feeding your computer? Do you need a UPS or line conditioner? Voltage spikes or low voltage conditions can do nasty things to equipment). I'm much more likely to want to quickly check what a given file looked like, say, three weeks ago, vs. two weeks ago, vs. yesterday, vs. today -- and that's something Time Machine can handle quite easily, while SuperDuper can't, unless you've got a whole stack of backup drives and you're
extremely diligent about rotating backups through them.
FWIW, FireWire is a better interface than USB for hard drives (FireWire was designed to handle things like hard drives, while USB was designed more for things like mice), though USB works well enough these days. Given the choice, I'd go with FireWire (on paper, USB2, at 480Mbps, looks faster than FW400, at 400Mbps, but FireWire can actually
attain those speeds, while USB usually can't, plus USB puts a much heavier load on the CPU to do that same transfer). And FW800 eats USB2 for lunch
This article is old, but still a very enlightening read, about how difficult it is to really get backups right:
Mac Backup Software Harmful (and to give away the punchline,
before Time Machine came out, the only software that
really got backups right was
SuperDuper).
As far as drive space goes, at work, I've got a MBP with a 320 GB drive, with about 115 GB used, backing up with Time Machine to an always-on 500 GB external drive that's about 2/3rds full now, with backups going back to the start of the year. Previously, I had backups going back about a year and a half on the same drive, connected to my previous workstation, but I cannibalized most of those when my machine got replaced (I kept a few "key" backups of the older system around, freed up most of the drive for the new system to use). On one hand, the minimum space you need for Time Machine is, "a size equal to the amount of space you're actually using on your drive, plus some more", because otherwise Time Machine can't do anything useful. But you'll be happier with something more like "3 to 5 times what you're actually using", so, say a 300-500 GB drive if you're currently using 100 GB. That'd give Time Machine lots of room for copies of things, so you'll end up with backups going much further back in time. Then again, 1 TB drives aren't that expensive these days... And if you're serious about backups, and can afford it, what I'd really like to have at home for Time Machine backups would be a RAIDed NAS box from
QNAP or
ReadyNAS, but they
are spendy.
The way Time Machine works, it effectively stores only changed files for each new backup, so you can keep many MANY backups on the same drive, if most of your files aren't changing most of the time (e.g. from day to day, all of Mac OS X stays the same). If you've got 50 backups on the drive, and you have some file that has only had two versions during that time (e.g. the old content, before you changed the file a few weeks ago, and the new content, since then), then Time Machine will only have two copies of that file on disk (the old one, and the new one), for all of the backups -- not two per backup, just two copies total, with one linked into (roughly) half the backups and the other copy into the other "half". Thus Time Machine can be fantastically frugal with disk space. On the other hand, if you go through one day and, say, change the capitalization of artists names in all 20 GB of your music files in iTunes one day, well then that's going to chew up 20 GB on your backup drive on the next backup to save the new versions of all those files.
