Asus SCSI 320-2 - Manual (Page 18)

LSI MegaRAID Userʼs Manual
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5. This warning window will prompt out when hard disks of different sizes
are deployed in a RAID. If hard disks of identical space are used, this
window would not appear. (We recommend using hard disks of a same
model for RAID configurationn)
6. You could see the information of RAID in the Array Definition window.
Click Next to continue the settings.