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I just plugged in an external hard drive that has Firewire 800 and started Time Machine, but it seems that both the internal hard drive and external hard drive work frantically copying about 75GB of data from the iMac to the external hard drive.

And in fact, I dare not use the computer while it is copying that much data, as I don't want to over work the hard drive's head.

If it is the case of Time Capsule, backing up using Wireless, then won't it actually work better, because if Wireless-G is limited to 1 or 1.5MB of data per second, then it won't stress the hard drive, and user can work on anything without affecting the hard drive's hard work?

(Updated: Details: 1MB/s, instead of about 75MB per second if using the Firewire 800. In that case, the user can use the Mac and won't stress the hard drives, especially if the Mac OS X might read 50MB at a time, and put that data in RAM (as a hard disk cache), and just pass this data to TIme Capsule by Wireless-G -- then there is no need to read any data again until 50 seconds later, if assuming the copy speed is about 1MB/s. Would this be true?)

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That last paragraph is very difficult to understand, but if I follow, creating a RAM disk is not going to help...primarily because there's no problem to fix in the first place; Your concerns are unwarranted.

Time Capsule hard drives in particular are very low failure rate, and above that, Time Machine does not tax your system indefinitely. When the timer starts a new backup, the hard drive will spin up. On completion, it will spin down as necessary. These drives, like most of decent quality today have a high MTBF & MTTF, so relax.

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no, it is simply this: since Time Machine can back up 1MB/s if using Wireless-G, then the source hard drive won't be reading crazily, at 80MB/s to keep up with the back up. It merely needs to read 1MB/s, and for that matter, it can read 50MB first, and then the OS will automatically store that 50MB of data in RAM, and then provide to the back up destination for the next 50 seconds. So the source hard drive will be really relaxed, instead of being stressed to provided 80MB/s and making the computer not convenient to be used during that time –  動靜能量 Nov 4 '11 at 8:08
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Assuming the process actually goes as you speculate, it would actually put more stress on the drive to use WiFi, because it has to spin up, read, spin down, spin up, read, spin down...and on for several hours at that pace. Not to mention the read head is moving all over the place if you are actively using the computer on top of it. Let it back up at (its) normal speed and walk away while its doing so. Backups going forward should be relatively small and then this becomes a moot point. –  tubedogg Aug 25 '13 at 4:34
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There is nothing to worry about. Hard drives are designed to work in high-load environments. Furthermore, that data access is not very common (most backups will be very little data after the initial backup completes) and rather elastic - either you're reading 50MB/sec for a minute, or reading 1MB/sec for fifty minutes. Both produce an similar amount of "work" for the drive.

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