Welcome to A.org :-D
This is a common problem on 3.0 machines like mine. Basically, the Amiga boots without giving enough time for the HDD to spin up. It cannot detect it, and therefore asks you for a disk, but if you do a soft reboot, the drive is detected and then boots - its more of an annoyance than a real issue. I've no idea if this problem can occur with other kickstarts, and if there is a different reason maybe someone could update the FAQ I wrote
here?