I can get a 320 GB drive on sale today at a pretty good price. It will match the size of the 320 in my iMac and will be used as a clone backup drive. However, it is only 5400 rpm. The question is: Do I care if it's only for backup and limited applications use?

My backup M.O. is to use Super Duper, allowing it to make the clone and shutdown afterward, so it's not like I'm in a hurry for that part.

However, I use the clone as a boot drive when running applications like Disk Utilities, Disk Warrior or TechTool Pro. There will never be anything as intensive as games et cetera. Are the utilities' performance likely to be affected enough that I'll be wishing I had 7200 rpm?

EDIT: Updating the information....I was just checking this drive on-line and see that it's USB 2.0 (although it says upgradable to FireWire 800...without saying what that means). I currently use an 80GB drive I recovered from a previous iMac and it's on Firewire.

This is strictly my personal opinion but if you calculate the cost per GigaByte of data storage, a small drive with an apparently low initial cost will inevitably turn out to be far more expensive per GB than say a 1 or 2 TB drive. Do the numbers.

The low cost of the drive you are looking at is reflected in the fact it is USB 2.0 which means it uses really cheap components in the enclosure. Not only that but a USB 2 High Speed has noticeably slower throughput than Firewire 400 much less Firewire 800. Lowest cost is seldom a bargain.

Since Apple recommends a Time Capsule that is two or three times larger than the drive you are backing up, you might consider getting a 2 or 3 TB firewire drive (I would say Thunderbolt but I don't know where to find one yet) to use as your Time Capsule and use that for your primary backup. Then take the 80 GB drive you already have, install whatever version of OS X you are using on it along with one or two good volume repair utilities such as Diskwarrior, TechTool Pro, or Drive Genius and use that as your emergency drive. If you are running Lion (OS X 10.7) you already have a Recovery HD partition on your HD that you can use to reinstall and/or recover all your data from Time Capsule. It is a pretty reliable system and I no longer feel any need to keep a clone — which was never up to date anyway.

How fast does your boot HD spin? (I found the info in /Apps/Utils/System Profiler > Contents > Hardware > Serial-ATA > Rotational Rate.)

If it's running at 5400RPM, your expected usage of the new one doesn't give you any particular reason to buy a 7200RPM drive (and the extra money would probably be better spent on more RAM).

Caveat: Wait for some input on that HD before you buy it...perhaps post a link.

You might want to review a previous thread, External hard drives for backups , in your deliberations.

To get directly to your question:

Lower rotational speed means lower read/write speed mostly for large files that are stored sequentially. if you are reading or writing many small files, the time it takes to move the drive heads is a much bigger factor than the speed the drive spins at. Even a Ferrari can only get through downtown Manhattan at rush hour so fast; it's not going to be any quicker in rush hour traffic than a Chevy Lumina!

I use Carbon Copy Cloner, and I have observed that it writes files to the backup drive in small chunks. I have two backup drives--a 5400 RPM and a 7200 RPM. (One of them I back up to often--once a week or more--and the other I do backups less often, but keep in a fireproof box in case the entire house should suddenly fall to some unfortunate disaster.) I have not noticed even the slightest difference in how long it takes to do a backup to either one.

Thanks for taking time to make a few good points. They forced some thinking.

However, the size of the Time Capsule is not a big issue for me. That is, I never need to go back very far and find that 80 GB (currently on a LaCie drive) is more than adequate. There's a bunch of non-critical stuff I instruct Time Machine not to record as I will have it on my clone, and I do my clones weekly.

A clone drive, similar in size to my main drive (320GB), should also be adequate because I still only have 119 GB after many years., and a big chunk of that is from keeping a few movie backups. With a new clone drive I'll now have a freed-up 80GB drive that can take that load....further reducing capacity requirements on the main drive.

I've looked around and OWC has a 320GB unit that looks pretty good and has good reviews. It contains a Hitachi drive.

My boot drive is 7200.


Done, good advice....I'm passing on the 'on-sale drive' due to the USB interface. I'm already FireWire so it doesn't make much sense to take a backwards step there.

Continuing with your traffic analogy I assume that, in this case, the USB connection is the road crew with jack-hammers who have erected barricades reducing four lanes to one.

Heh. That's awesome.

That would be the case with USB 1, but honestly, I really doubt that a Carbon Copy Cloner backup will saturate USB 2. It's spending so much time reading small chunks from the internal drive and then sending them down the pipe to the external drive that the pipe is never going to get totally full.

It most certainly will. When it encounters a large file to back up and determines it doesn't exist or will be replaced entirely, it will push the drives to their limit on speed. Expect 18-39mb/sec depending on your USB bridge chip. 18, 26, and 36 MB/sec are the speeds of the most common sata to usb bridge chips, I've only seen 39 on OWC brides and I'm not sure how they're pulling it off.

I watch large file backups quickly flat top activitymonitor here at 79mb/sec on firewire800.

The big thing though with backups is that you don't do that very often usually, and backups typically run during off-hours like automatically at 2am, where speed isn't too important as long as it gets done before morning. So a slow drive, or even on a slow connection or even over the internet isn't a big deal.

I don't personally feel a faster hard drive matters much, but I know a few people that insist on faster drives in their mbp's for boot camp and vmware/parallels use, apparently it helps quite a bit there going from 5400 to 7200. Though even I wouldn't turn down a cheap large SSD