Disk performance expectations
So what are the reasonable expectations for how your disks should perform? The last example shown demonstrates how things should work. Any good drive nowadays should have sequential transfers of well over 50 MB/s on its fastest area, with 100 MB/s being easy to find. The slowest part of the drive will be closer to half that speed. It's good practice to try and test an individual drive before building more complicated arrays using them. If a single drive is slow, you can be sure an array of them will be bad too.
The tricky part of estimating how fast your system should be is when you put multiple drives into an array.
For multiple disks into a RAID1 array, the sequential read and write speed will not increase. However, a good controller or software RAID implementation will use both drives at once for seeking purposes, which might as much as double measurements of that rate.
When multiple drives are added to a RAID0 array, you should get something close...