How big a hard drive do I need for iMac and Time Machine?

Question

Sorry if this has been answered elsewhere already. I am going to buy an iMac, probably the new 24" with a 320G internal HDD. I already have one Lacie external 320G HDD so I was wondering whether I can use this for Time Machine backups, or do I need to buy a bigger one? If so, how big? Is there a rule of thumb for working out how much hard drive space you need to have for Time Machine, for instance if I were to buy the 500G iMac what would I need?
thanks.

Answer 1

I agree with the "double the size" rule of thumb. You should also consider how much of the 320GB you are actually using and are likely to use in the next year or two. After all, it's the amount of storage you use that matters, not the size of the disk. Disks will very likely be cheaper soon. You should also keep in mind that TM deletes old archive data when it runs out of disk space. So as long as you have more disk space than you have data to back up, TM will work, though you will lose older versions of your data.
So if your 320GB drive only has 100GB used and you're not the kind of person who eats up disk like there's no tomorrow, the 320GB drive should be fine for now. If you expect that 320GB disk to be full soon then a 750GB TM disk is more like what you need.
The same logic applies to a 500GB iMac. If you only expect to use 250GB in the next year or two then a 500GB TM disk should be OK. A 1TB drive will be a lot cheaper next year or the year after.