I-Mac will not boot

[RX10]

When pressing start button, white screen appears then the apple logo alternates with a circle with diagonal line through it.
I had just bought a small 500Gig external HD to back up photos. It had to be formatted, not sure I did this correctly. Downloaded photos and quit the drives. Now cannot restart. The I-Mac is about 7 years old.
Any suggestions please.

 

[Spawn_Dooley]

which Mac OS X version?
Boot holding the Opt key to see what happens.

 

[RX10]

Thanks so much. Just started with holding Opt key, main drive appeared and after selecting it the machine started as normal.
Thanks again for your advice.

 

[Spawn_Dooley]

That's great RX10