[SOLVED] Identical SSDs, one underperforming. Why?

[tobleroneII]

P9x79 pro Motherboard
2x Crucial MX500 2TB in AHCI (both in SATA 6Gb/s)

One has a read/write of ~500 MB/s and the drive is 50% full
The other has a read/write of ~250 MB/s and the drive is 25% full

What could be the cause?

 

[USAFRet]

P9x79 pro Motherboard
2x Crucial MX500 2TB in AHCI (both in SATA 6Gb/s)

One has a read/write of ~500 MB/s and the drive is 50% full
The other has a read/write of ~250 MB/s and the drive is 25% full

What could be the cause?

P9X79 PRO - Support

www.asus.com

Intel® X79 chipset :
2 x SATA 6Gb/s port(s), white
4 x SATA 3Gb/s port(s), blue
Support Raid 0, 1, 5, 10
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"~250 MB/s " indicates that drive is probably connected to a SATA II port.