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My backup drive (named seagate) kept getting the error- not enough space - although the size of the backup is the same as the HD. I am at 60% capacity on my HD.

The error continued even though I erased my backup to restart the process (as well as deleting unnecessary files from my HD, and limiting what was being backed up)

In setting up Time Machine again, it's asking if I want to replace the back up disk "Macintosh HD" or back up to both disks. I just want to use seagate, but am now concerned somehow it will erase my HD. Is that concern invalid? It would certainly be counterproductive to lose what I'm trying to save

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From your description, it sounds like you have an external hard drive (Seagate brand), which you have been using for your Time Machine backup. You're also saying that the capacity of the external hard drive is the same as your Mac.

As a general guideline, you want the Time Machine's hard drive to be about 1.5x the capacity of the Mac's hard drive. The reason being is that Time Machine stores progressive changes to files, such that you could literally restore one version of a file from a week ago, even though the file had been backed up several times over the week.

With respect to setting up Time Machine again, it sounds like you have a Time Machine backup on the external hard drive, and you're being prompted to restore. If this is confusing to you, then you could simply format the external hard drive, and then designate it for your Time Machine backup.

For all things related to Time Machine backups, and troubleshooting issues related to that, take a look at Pondini's Guides and Tips for Time Machine.

  • Like the rest of your answer, but I'd go for double or even bigger for the TM drive. Mine's double & it really doesn't have much history. I'd prefer at least 3 months' data, if not more - right now it only goes back 6 weeks – Tetsujin May 29 '15 at 17:16
  • It mostly depends on how much you're backing up. Some people might make gigabytes of changes in a day, others barely make more than a few megabytes a day. The first guy would probably need a drive a few times the size of their internal one to get much use out of it. There's also the issue that Time Machine backs up entire files, rather than only the changes to those files, resulting in a massive amount of redundant data. – William T Froggard May 29 '15 at 17:19
  • One's mileage varies according to their needs, such as the difference between casual versus professional use. Minimum rule-of-thumb is 1.5x the capacity, though it doesn't hurt to have excess capacity. It also wouldn't hurt to have external drive with dual-drive storage in a RAID 1 configuration, so that there are always two copies of the Time Machine backup (i.e., in case one of the external drives fail). – Vzzdak May 29 '15 at 17:35
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Are you certain that there is enough space remaining, even after all you've done? Look at what Time Machine reports as its backup size by clicking "Options" in Time Machine preferences, then compare it to free space remaining on your backup drive.

Removing a backup disk doesn't do anything to the data. You can even re-add it later if you want to.

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