Newbie question

[chris_johnsen]

Originally Posted by vizcard

thanks for getting back. I still don't know how big of a external hard drive i should buy.

If my Macbook HD is 500GB, how big of a external hard drive would I need just for a SuperDuper backup.

Like Dave said, you will “need enough space for the data plus plenty of extra growing room”.

Technically, if you are only using 40GB of your 500GB disk, you could back that up onto a 40GB disk. But if you ever start using more space, your backups will run out of disk space before they finish. Or if you are using Smart Update and there is not enough room to cover for your file "churn", then the backup will also run out of disk space (for safety, Smart Update copies new files to the backup drive before it deletes old files from the backup drive; so if there is lots of data churn—either many small files or just a few large files—Smart Update can still run out of disk space while copying 40GB of data onto a 40GB volume).

If you have very little churn, then you could try to get away with just 10% extra space for Smart Update's copy safety issue. If there is lots of churn, you may need as much as 100% extra space.

As I understand Time Machine, it will take as much disk space as you can give it. You'll need at least 100% of your data size, plus as much extra as you can afford for the extra files copies. Again, you can try to guess about this using the idea of "churn". If you are making changes to many files (or changes to large files), then you will probably want more disk space.

So for a drive that will hold a SuperDuper! backup plus Time Machine data, you might want to go with at least 310% (100% for the backup, 10% for Smart Update, 100% base to Time Machine, 100% for Time Machine extra file copies). If you have more "churn" you will want to increase both the Smart Update percentage and the Time Machine percentage.

For SuperDuper!, you should generally use a volume that is a bit larger than your data. For Time Machine, you will want to use the biggest disk you are willing to buy for the purpose (probably at least 200% of your data size).