Raspberry pi stop disk spin down

[Jim]

I have a hard drive attached via a USB adapter to my raspberry pi. The 
problem is that the disk always shuts down after a few minutes of 
inactivity.  When I want to access something on the disk or write to it, 
I have to wait for it to spin up.  How would I go about setting it so it 
doesn't spin down?  I tried to look online and found sdparm mentioned, 
but found nothing I could understand. Please help. Thanks