You'd be greatly lowering the possibility that you would loose all your data. To give you a crude example say both drives 1 and 2 have a failure rate of 1/1000. In Raid 1 you calculate the chance that both drives would fail which would be
(1/1,000)*(1/1,000) = 1/1,000,000
So you would go from a .001 rate of failure to a .000001 rate of failure for the drive array since for it to fail completely both drives need to fail at the same time. As soon as a single drive fails you just need to replace it to rebuild the mirrored array. Write speeds with medium sized files shouldn't be too affected so long as the drives have a decent amount of cache. Read speeds will be improved though.
Because of cost RAID 1 isn't really worth it IMO, given the price of HD, RAID 5 would seem like a better choice (but requires more sophisticated controllers).
If you want a MB to do RAID, you simply look at the "SATA RAID" spec under the "Storage Devices" category on newegg.
Writes might be a bit slower, because the controller has to make sure the same thing was written on both drives. How much? No idea
What RAID 1 does is simply to replicate data, if 1 disk fail, you still have the other. In case of a power surge, if both drives were on the same channel that got the surge, chances are they will both fry, but if by chance one was a bit more tolerant ... you are saved .
The value of raid-1 for protecting data is that you can recover from a hard drive failure quickly.
It is for servers that can't afford any down time.
Recovery from a hard drive failure is just moments.
Fortunately hard drives do not fail often. Mean time to failure is claimed to be 1 million hours or so. That is over 100 years of 24/7 operation.
Raid-1 does not protect you from other types of losses such as viruses,
software errors, operator error, or fire...etc.
For that, you need EXTERNAL backup.
If you have external backup, and can afford some recovery time, then you don't need raid-1.
RAID 5, just like RAID1 is for data security, but I think it might be even slower on writes because of parity or checksum calculation used. Basically RAID 5 requires at least 3 drives, but you only "loose" the equivalent of 1 drive of capacity (where in RAID 1 you loose half). You could simplify RAID 5 as being some kind of hybrid between RAID 0 and RAID 1.
I still don't fully understand what RAID 5 is. Is it like two drives in RAID 0 and a third drive as a backup? The descriptions on websites are very vague. Also, with 3 drives needed, how would this be a better choice?
RAID 5 is far more complex than just a backup. Each drive holds it's own data and "recovery data" for the other drives. If one fails the array can still be "reconstructed", but it takes far more time than with RAID 1. However, when using RAID 5, you get ((N-1)/N)% of usable space where RAID 1 is only 50%; you would be able to use 66% of the total space of a 3 disk array, 75% for 4 disks, 80% for 5, ... you get the idea.
IMO RAID 5 is a bit better than RAID 1 because the data writes are splited between disks (like with RAID 0), but also offers security which RAID 0 lacks.
Anyway, as geofelt said, RAID 1 isn't a backup, if you get a virus, the virus gets duplicated on both disks, you accidentally delete a file ... gets deleted on both disks at the same time. RAID 1 is only to prevent from physical failure. Since I never had any drive die on me, I don't see any real advantage for going RAID 1, at least with RAID 5 you get a small boost in performance for larger files ...
Zenthar suggested that RAID 5 would be a better choice. What's the difference?
Is there a way that you can put a mirror of your drive onto an external hard drive? By mirror, I mean the OS and everything. How would you do that? I have a 250GB external I could spare 40GB of.
Mirroring to an external certainly is possible, and it might be free. These days when you buy a new HDD, its manufacturer will give you several software tools to help set it up. If they don't come with the drive (because you buy a bare OEM unit) you can download for free from their website. One type of tool will Partition and Format your new drive, then copy to it from the old drive absolutely everything, making the new one a bootable drive that you can use in place of the old one. Now that's not really what you want, but it's close.
Most of these software freebies will only make the copy TO a drive from that manufacturer. So you'd need such tools from the maker of your 250 GB external drive to make the mirror backup copy. Then you might need a similar tool from the maker of your existing 80 GB drive in order to do a restore when needed. Alternatively, you can buy 3rd party software like Acronis or Norton Ghost for this purpose - those can work with any maker's drives.
For your purpose, Ghost and maybe Acronis can make a complete image anywhere on the external drive and give you the way to restore it later. The freebie software, however, works best if the mirror copy is made to its own separate Partition. So on your existing external drive you would have to free up space and create on it a second partition of up to 80 GB that the freebie could use to make its mirror image. To do that you might need something like Acronis or Partition Magic anyway.
One big gotcha, though, is that you have to discipline yourself to make the backup images! You can and should do that often - in fact, with something like Ghost you could keep two or three generations of backups in case the latest has the same problem you're trying to recover from. But you have to do it, and there are people like me who don't get around to it often enough!
For that reason in part, I set up a computer recently for our retail store's Point of Sale system that includes a RAID1 array with two 160 GB drives. (The software and data files for this application are small.) So if one drive fails the whole thing keeps running and it's supposed to be easy to replace the failed one and restore the array. For a backup system I bought and installed WinZip Pro, then set up an automatic scheduled backup that runs every day at 11:55 pm and Zips all the data files into one .zip labelled for that day and stores it on the drive array. About month-end (maybe should really be more often) I take my little flash memory "thumb drive" and copy the latest .zip file to it, deleta all the old .zips to free up space, then copy from the thumb drive to the HDD of another computer off site.