Do you have a raid option in the bios?
You can run in raid mode without being in an array. Any single drive in raid but not part of an array will default to achi mode.
only IDE and AHCI
The HP Pro laptops are a nightmare, most have somthing called "HP Driveguard" which is probably what is causing the issue you are having. I doubt there is a way around it and there is a fair chance when you put the original hdd back in, it will not boot due to this 'feature'. If this is the case then the only way to get the machine working again is to send back to HP.
No, it isn't problem with DriveGuard, I think. I can install system on 520 in IDE mode, and with HDD everything is okej, but only in AHCI mode ssd make my notebook stuck at hp start logo.