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Do yourself a favor and make a partition and install it on that... Way easier and if you don't like it, just delete the partition and take the space back on your main drive
 
This falls under the: if you don't understand or know the answer, then you probably shouldn't be installing beta software.

When using a beta you should keep at least 2 backups, one being 10.10 with up to date user data.
 
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