You already made a raid 5 in your hardware raid card.
Then you don't need to make a soft raid in system level.
According to your messages, however, your raid card is not set properly.
The OS should only see a single 750GB harddisk instead of 4 x 250GB hdd.
e.g.
Total available size = 250GB x (N - 1) = 750GB

If your raid card cannot be detected while installing, you will need to provide a linux driver for it.

楼主的安装步骤出问题了，建议你去参考一下正确的安装步骤或是自己多试几次，我就是自己多事了计财最后才成功的