All you have to do is manually copy your important drive to your macbook or other mac computer and when the time machine takes the backup it will be stored on two devices. You can then go back to your important drive and delete the data that you copied to your mac. This way you have more space and your data is still going to be stored on both devices.

I hope I answered your question.

Thanks. Tech from http://www.computerstar.ca/data-recovery-toronto.html

All you have to do is manually copy your important drive to your macbook or other mac computer and when the time machine takes the backup it will be stored on two devices. You can then go back to your important drive and delete the data that you copied to your mac. This way you have more space and your data is still going to be stored on both devices.