installed SSD in 2011 imac. Any reason to keep external Time Machine drive?

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I installed a secondary Samsung 840 Pro SSD drive using the OWC guide, cloned the original Hitachi HD that came with the iMac using Carbon Copy, and made the Samsung 840 my boot up drive.

If I use the older Hitachi HD as my Time Machine backup, is there really any reason to keep using an external drive for back ups? Are there issues I should be aware of when going from SSD main drive from a HD?

Just curious to know about those who have done in this regard.

 

[blueroom]

A TM backup (or any backup) should be external to the machine. Lost, damaged or stolen your backup's gone.

 

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So what should I use the older internal drive for? A redundant/backup startup disk?