Hi... can i use the time machine that you set up for old macbook for new imac?  Do i just plug it in and will back up this computer without erasing other backups from old computer?

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Yes, you can use the same drive to back up multiple macs using Time Machine. As long as there's enough free space on the external hard drive to copy all the other mac's data over you should be fine. Just go to system preferences/Time Machine and select that disk as the backup. It may ask you automatically if you'd like to use that disk as your Time Machine disk if you don't already have Time Machine setup.

Hope that helps.