SATA cables are the current way of connecting hard drives to a mainboard. They're flat, about 1/4 " width, and especially if you have an older mainboard, they tend to come loose rather easily.

There are several ways of operating two drives "as one": JBOD, also called spanning (which means simply daisy-chaining two drives in to one large volume), RAID 0, also called striping, which spreads the data over the two disks, speeding up transfer rate, and RAID1 (mirroring), which writes all data to both hard disks, so that if one disk fails, all data is still accessible on the other disk.

JBOD is of little use with the current 1 TB drives, those disks are large enough all by themselves.
RAID0 improves transfer rate, but if one of the disks fails, all data is gone. Quite risky.
RAID1 provides redundancy, which sounds like it's sort of an online backup system.

There's a couple of problems with RAID1. First of all, when setting up a RAID1 array, you'll probably buy both disks at the same time. When one of those disks fails, it's rather likely that the other one will fail too, within a couple of weeks, or even days.
Second - and this applies to most modern consumer mainboards - the single point of failure now is the harddisk controller, i.e. the southbridge chip. And especially nVidia southbridge chips are more susceptible to failure than the disks themselves. Case in point: I fried three nVidia southbridge chips on fairly new (less than 2 years) and even brand new mainboards within two weeks!

Concerning your problem: this may be a driver issue and/or a hardware issue.
If you can tell me how you "linked" your drives (either in the BIOS setup or in Windows using mount points), and if you can tell me the brand and model of the mainboard and the drives, I may be able to pinpoint the problem.

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