As this is a new build, check the cheap and easy first.
SATA cables can drive you nuts in trouble shooting. They can appear to be attached, but not fully attached or may have come loose during some poking and proding as you went along.
A small cut from a sharp edge in the case while tucking it away could also cause a problem.
Suggest you try:
Disconnect and reconnect the cable or replace the cable.
Use a different SATA port on the motherboard
I also noticed an improvment to performance of platter drives by changing BIOS option from IDE to AHCI during a SSD install. Crystal disk mark showed and improved performance number from previous IDE benchmark results.
IDE Crystal DiskMark results
Sequential Read : 91.467 MB/s
Sequential Write : 87.171 MB/s
Random Read 512KB : 26.938 MB/s
Random Write 512KB : 45.231 MB/s
Random Read 4KB (QD=1) : 0.287 MB/s [ 70.1 IOPS]
Random Write 4KB (QD=1) : 0.676 MB/s [ 165.1 IOPS]
Random Read 4KB (QD=32) : 0.493 MB/s [ 120.4 IOPS]
Random Write 4KB (QD=32) : 0.697 MB/s [ 170.1 IOPS]
Test : 1000 MB [C: 16.6% (232.6/1397.3 GB)] (x5)
AHCI enabled CDM results
Sequential Read : 105.395 MB/s
Sequential Write : 102.510 MB/s
Random Read 512KB : 29.256 MB/s
Random Write 512KB : 46.686 MB/s
Random Read 4KB (QD=1) : 0.304 MB/s [ 74.1 IOPS]
Random Write 4KB (QD=1) : 0.748 MB/s [ 182.7 IOPS]
Random Read 4KB (QD=32) : 0.359 MB/s [ 87.8 IOPS]
Random Write 4KB (QD=32) : 0.754 MB/s [ 184.1 IOPS]
Test : 1000 MB [E: 16.1% (225.3/1397.3 GB)] (x5)
Get a benchmark score from crystal diskmark or the like just to get some numbers to start with and see if you notice an improvment after the cable suggestion.
I really suspect the cable is at fault or the connection of the cable.