Can't startup, it asks to initialize hard disk

I'm getting that flashing question mark.
I had this problem before with another HD, but I could put that HD as a
secondary drive into another MAC and was able to access all the files.

I'm getting this flashing question mark with another drive. Its a 6month old
drive. With this problem HD, running disk utility shows the HD not mounted.
When I put the HD into another mac as a secondary drive, it won't recognize
it. It keeps asking me to initialize the HD.

I backed up most of the files, but would still like to take one last look.

Any ideas?

Thanks