Drive size depends not only on what you need for storage but on what the System/OS can address, and on the drive's format. Large drives with HFS can be extremely wasteful of 'space'. I was transferring files from an inherited (with a 9600) 18GB drive formatted HFS(!) recently. Min. file size was 278kB, by contrast with the 2-16kB that you will get with HFS on smaller drives that are contemporary with the Systems that you are considering. File size can be a fixed min. 4kB under HFS+, but OS 8.1 and up is needed to cope with HFS+.

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