Sorry for using a bit too much technobabble... The traditional Mac file system was called "HFS" and dates back to a time when a 10 MB harddrive was expensive high end gear... :-) It used a fixed maximum number of blocks which was suitable for disks up to the size of a CD, but wastes precious space on larger ones - officially, the recommended limit was 2 GB (!) "HFS+" was the follow-up system introduced with OS 8.1 which has a flexible block size suitable to the specific disk size in question. If you format any drive, you will get a popup that asks you for "Mac OS Standard" or "Mac OS Extended" - this is just "HFS" and "HFS+". You can tell for your disk(s) by the "Get Info" window, it is displayed in the "Format:" line. It would be a very bad idea to format your 20 GB drive with "Standard", because even small files will take huge disk space then. The problem might be your "emergency" boot CD. Under OS 8.0.x (or earlier), "Extended" disks will not be readable - if you do so, the only content is a "read me" file that tells you not to panic... the files are there, but you need OS 8.1 (or later) to see them. This means, if you only have a OS 8.0 boot CD, and then installed the 8.1 update (there is no 8.2), formatted the disk as "Extended", and one day your system on disk completely crashes, you might be in trouble: You can't boot with the CD and re-install the system (unless you re-format the disk and lose all the files). So, if you have no chance to get a real OS 8.1 boot CD, you should always format the startup harddrive as "Standard". In the worst case you can then re-install 8.0, run the updater, boot under 8.1 and can read other disks again.