The Intel Raid is managed by the mainboard and/or driver. lvm tools dont even get to see the things.
Your Linux seeing sda and sdb means it saw through the raid setup of the mainboard, which is a bad thing (tm).
There´s several levels in a raid: 1) the hardware 2) what the raidcontroller makes of it 3) what the OS sees. In any reliable raid system, 2 and 3 are the same. If they arent the same, questions like yours arise, confusing even the most seasoned admins. In this case, it looks like you got lucky. You did the wrong thing, your raid setup ignored you, and now is doing (hopefully) the right thing.
This is not always the case. Equal chance is, you do the right thing, the mainboard raid ignores you, and does the wrong thing.
The only way to securely repair any kind of raid is through the tools of the raid system.
What the Intel driver is now doing, is a dd, calling it rebuild. Of course, it didnt see what your dd did! It doesnt have an idea where the data output of dd comes from, and cant now that it is, in fact, the correct data. So it has to do the copying itself. For all the poor thing knows, it could be grandma´s collection of turkey recipes.
For any good solid proper raid setup, things have to be deterministic. Mainboard raids usually arent (BIOS version, driver version, OS, etc). The admin has to train him/herself to repair the raids. If you put any kind of important data on a raid, you must work yourself through some of the failures of it. If you dont, you´d probably be better off without a raid. Turns out, most of the time, only OS software raids, or raid card raids are deterministic. The mixup of mainboard/driver raid that almost each board has is not much more than a placebo.
P.S. do you have a backup?
If you mean the RAID controller built into the motherboard, I'd AVOID IT. It's not true hardware RAID.
Motherboard RAID is regarded as the worst of RAIDs, as it is motherboard specific, there are several online instances of the motherboard just losing the RAID configuration and hosing volumes, and in the end, if you're trying to get RAID on the less expensive but capable side, use software RAID built into Linux.
True hardware RAID is cached and will cost you in the wallet, but it costs more for a reason. Motherboard RAID often is just software RAID in firmware, only it can make the volume specific to that machine. Drive die or hardware issue? You can't necessarily recover the data by moving it to another system, since the motherboard may have done something odd to the formatting of the disk volume.
If you're looking for hardware RAID with Linux, I've had good luck with 3Ware controllers, and if you don't want to spend the cash, use software RAID. Comes free with Linux.
Best Answer
I personally suggest to set drives as AHCI and use only linux mdadm.
The reason is that if you move your drives to whatever other server, you just use the same mdadm utility and you don't have to care wheras there were some "compatible" metadata before or not.