Replacing a predicted to fail SAS drive in a Dell MD1220

The A and B in sda and sdb should map to channels 1 and 2 (or 0 and 1) for your drives. If the system is set up so that they're labeled, you can tell that way. I don't know how your drives are structured with the wiring; I've had them numbered with small print on the motherboard so you can tell what port is going to what drive.

I supposed you could use your idea to then try feeling for vibration from the drives too, if there's enough room for you to feel the drives. Again depends on the way they're mounted.

No, and if you have 4 disks and want two spares you are better off performance wise with a 4 disk RAID 10 instead of a 3 disk RAID 5 + hot spare. Either way your array will have 2 disks worth of usable capacity but the RAID 10 will have better write speeds and takes less time to recover from a failed disk.

Assuming the RAID 5 doesn't contain your boot volume, I'd go ahead and backup the data, reconfigure to a RAID 10, and restore the data.

    RAID Level Comparison for arrays up to 8 drives.
Features                RAID 0  RAID 1   RAID 5   RAID 6   RAID 10
Min Drives                2       2        3        4        4
Data Protection           0       1        1        2        2 (Up to one disk failure in each sub-array)
Read Performance         High    High     High     High     High
Write Performance        High   Medium    Low      Low     Medium
Degraded Read Perf       N/A    Medium    Low      Low      High
Degraded Write Perf      N/A     High     Low      Low      High
Capacity Utilization %   100     50      67-87    50-75     50