Unable to install Windows 2012

Create two logical disks on the array, one small one less than 2TB for the OS and one or two large ones for the data.  ProLiant gen8s do not support UEFI so cannot boot from logical disks greater than 2TiB.

Which model specifically, sounds as though the hp array 420 drivers are not presented/slipstreamed into the OS.

Ahci might not really matter since you are booting from the RAID controller versus the mb sata ports.

Download the hp array 420 drivers for server 2012
When at the storage prompt, ho through advanced to load the raid controller drivers. Then rescan at which time you shoukd see the logical volumes you created as disks.
Partition the first disk as suggested, 200-300gb for OS Install.
You can setup the rest after OS install and I stall of HP insight

Don't need to download drivers if you use Intelligent Provisioning, (F10 prompt), it puts them on for you.

@andyalder, agree, but the user says that there is an issue, one option, the intelligent provisioning is not up to date .....

sounds as though the hp array 420 drivers are not presented/slipstreamed into the OS

if the array drivers weren't in there, i don't think windows would say it can't be installed to that disk because it wouldn't see it to begin with
the G8 was released after server 2012 was so likely the drivers are there
i have an unused Gen8 at the office that (i think) has a 420i so i could test monday if this is still an issue

Need to see what the asker sees when trying to deploy.
If the disks are presented, the asker would need to use the advanced option to partition the volume presented into a smaller partition as was raised earlier dealing with the 2TB limit.

Thanks for the comments. Not a driver issues, the drives show up, just when picking that partition to install to, i get that message...

Thanks,

How large do the drives show as? And how many of them?

8x1.2TB sas drives,
RAID 5,6,10?

from 8.4TB raid 5
7.2TB  RAID 6

4.8TB RAiD 10
make sure you are not creating partitions manually and pointed the install at too small of a partition.
i.e. recovery partition 20 GB main partition 200GB and you mistakenly chose the 20 for the install.
The other make sure the partitions you create are primary and not logical.../extended

Doing RAID1 for two of the 1.2TB, formating for smaller drive size. Just tried a 300GB, got the same message, unable to boot from that partition...

Please post the partition layout, and the message
something is being overlooked.

Are you using IP to do the guided install?

What was it set to? Default is logical disk 1 on array A.