If it's a newer motherboard and case, there should be jacks on the motherboard itself for the fans . It should be safe yes. There are minute reasons why you might not want to, but the odds of interference or power supply variance are quite remote.
All of those power cables are connected together in the power supply anyway, and neither a fan or hdd uses much power relative to what the wire can carry. I've always paid more attention to a neat physical layout for good airflow and never had a problem with what was on the same power lead as what. The hard drive should have it's own EMI filtering in it, and a brushless fan doesn't generate noise like a regular dc motor.
I'd have the on a separate circuit. The hard drive needs all the power it can get to act well
It shouldnt hurt anything. but i would rather have mine on a separate circut.
Well it is always better to seperate circuits when they are related to hard drives even data cables as hard disk take loads of data bus and power.
