how to copy data from a failed a raid drive?

I agree with the above, but it is bad practice to even consider having to recover data from a failed drive (and I wouldn't even know why anyone one would want to recover data from an OS drive, as that doesn't really contain any data of value, that would be on separate volumes). You should really only rely on your backups and DR schemes and be prepared.

Understand, RAID IS NOT BACKUP.  If you're treating it as such, STOP.  Get a good BACKUP solution in place.  RAID cannot protect you from disk corruption, accidental deletion, viruses, and malicious acts by employees. RAID is REDUNDANCY.  It allows for multiple copies of the data so that if one drive fails, you don't lose everything.  If the entire RAID fails (which, in theory is less likely than a drive failure or your wouldn't be using RAID) then you would potentially need to use software such as the software recommended by dlethe. (I've used it to recover a couple of RAID 5s in the past).  It's NOT fast and doesn't ALWAYS work, but it's very good for what it is.

I should also clarify my answer.  I was replying from context that HDD doesn't boot the O/S, not that there is anything wrong with the HDD.  As somebody who is in the 'biz, my professional advice is that you have a HDD tester to assess health of the drive w/o spinning it up.  Unless you know what is wrong with the disk, spinning it up can risk further damage, to the point where you can make full recovery impossible even for the guys in the bunny suits.  

There is no  magic software that can fix a bad bearing, or head crash.  These things take labs.   If the data is worth $500 at minimum, then just don't even turn it on and take it to a pro.  This is is about as good as a generic answer you are going to get at least from me.  Data recovery should be left to professionals if you can't afford to lose the data. Odds are that shareware and consumer software can get most if not all of your data, the unfortunate reality is that this is not always the case.  You typically only hear about successful easy recoveries, not situations where people screwed up a DIY recovery and the data was lost.

What type of RAID array (0, 1, 5)? If it's a RAID 0 (striped) array, you'll need special software such as RAID reconstructor mentioned in a previous comment.

Also RAID 0 is more failure prone than a single disk.

I never said I use raid as a backup solution. I just had a question that it was so easy for me to copy all the data from a failed single hard drive and just wondering if there is a way I can do this with raid. For example, I only have 1 hard drive with c and d partition. Data are stored on D partition. If this drive failed, I could pull out the drive and connect to other computer through usb adapter and copy the data I need. So I was asking if there is something similar and easy for raid.

"If this drive failed, I could pull out the drive and connect to other computer through usb adapter and copy the data I need." -- not necessarily true. Many HDD failures are catastrophic and data can only be recovered by a data recovery firm. For other failures, data recovery software such as GetDataBack is needed. It's rare that you can just connect a failed drive via USB adapter and copy data. You can recover data from a failing drive by copying files in many cases.

Answer my question about what type of RAID array you have.

>> It's rare that you can just connect a failed drive via USB adapter and copy data.

So far, I have been able to make it when users not able to boot into windows or due to bad blocks... so probably the hard drive was not in a sever condition so I was able to make it.

The raid I referred to was raid 5. I guess there is no easy way as leew mentioned.

In RAID 5 you would need more than one drive in order to even think of recovering data. As leew and dlethe have suggested you need at least the RAID reconstructor for this, and probably also Getdataback. But as I mentioned you shouldn't even consider this, but rather make sure you have good backups. If you are thinking about this on behalf of possible customers, then educate those so they have a good backup in place. After all, a backup is much cheaper than recovering data, and the recovery is usually not completely successful.

what's the backup solution do you guys use? we have backup exec... but honestly.. I don't find it efficient

Since it's a RAID 5 array, can't you just replace the failed drive?

I use the backup included in Windows 7 Ultimate (also in Windows 7 Pro). Changed files daily and full image weekly to an external drive. Also have 2 HDDs in a mirrored (RAID 1) array to tolerate a single HDD failure.

Backup solutions is really a different question. And the answer depends on the quantity and importance of the data.