Here are some general recommended settings (no matter the board) you can use if you have some problems with booting your system or the card is not detected, and so on:
Set PCIe Link Speed to GEN2
Disable Internal Graphics (unless you need it for a monitor)
Enable Above 4G Decoding
Disable CSM Support
Ensure the boot priority is set correctly - storage device containing Hive as #1
(Optional) Restore AC Power Loss (Power On)
For ASUS B250 Mining Expert, PCIe Link Speed needs to be set in two different places:
Advanced\PCH Configuration\PCI Express Configuration -> PCIe Speed -> Gen 2
Advanced\System Agent (SA) Configuration\PEG Port Configuration -> PCIEX16_1 Link Speed -> Gen 2
And here a few additional tips on B250:
Some users report better compatibility with Gen 1 instead of Gen 2 on larger rigs
Some users report better stability with x16_1 slot kept empty
There's a "newer" bios available which supposedly solves a lot of previously reported issues with the board