What model is the replacement motherboard? I've got a Z68 motherboard with an Etron controller and have also noticed strange USB3 speeds with USB sticks but also a Toshiba Canvio 1TB external drive. On Windows 7 I'd always got decent write speeds. But yesterday I tried it in Windows 8.1 and was shocked to see speeds around 30 MB/s while transferring iso files. This was with the MS drivers so I installed the Etron drivers and speeds increased to around 64 MB/s. After rolling back the driver to the MS ones it was giving about 63 MB/s. This morning I brought the pc out of sleep and tried again. Speeds were what I used to get in Win7, around 110MB/s, which is what I've seen in reported benchmarks. Will try again later and also in Windows 7. But there does seem a lot of people are having this USB problem in Windows 8.1. Maybe it's mostly people with older systems with non-native USB3?
Holy wall of text batman! Nope, you can see my earlier speed tests here done on a Asus Z68 Rog with NEC/Renesas USB 3.0 Controllers, full speed. The newer Asus Rog Z87 with Intel is a touch faster but device is bottleneck not interface speed.
Sorry, was in a hurry to get out and fight crime. BAM! POW! The point I was making is the max write speeds were different almost every time. Didn't seem to matter which driver. Whatever it was, seems to have resolved itself for now. Really strange though.
Now i'm using MSI z77a-g43... But MSI website didn't tell what USB chipset they were using and they didn't gave the usb driver too... Help me pleaseeeeeeeeeeeeeeeeeeee....
Go to Specifications then click "Detail Specifications". Intel. It's a Z77 so has integrated USB3.0 ports. Intel don't even seem to have a driver for Windows 8/8.1 on their site either. I noticed Asrock only have USB3 drivers for the extra Asmedia ports. Is this normal for Series 7 chipsets? BTW what USB sticks are you testing this with? I bought a Patriot USB3 stick the other day. It gets about 5.5 MB/s write. Other people are getting similar. And it gets damn hot just plugged in and not even being accessed. You can laugh now.