Restore does not appear as a selection on f8 boot options

I have done a few restores already by just plugging in my external HD
with the restore points, then press F8. about 30 minutes later, voi
Now, it does not appear as an option. Is there a limit to the number
of restores you can do? Why would it suddenly no longer appear? Using
this restore saves me about 8 hours of installing all the software I
keep loaded, so I really need this.

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