There's no reason whatsoever that the 7200RPM drives won't work, unless it's a firmware issue. Systems normally don't bias against drive speed, only drive firmware. And even then, rarely. (I wrote an article about it...

Try what I suggested on one of the drives, and report back.

No mention of eMacs biasing against drive speed.

This also affirms that 7200 RPM drives are compatible.

I'll put money it's your drives formatting. Follow what I suggested.