When a RAID 1 array performs a resync
is it actually copying all the data
from one disk to another or is it just
analyzing the data on the disks and
correcting any differences?
With a two drive RAID1 array, it is hard to 'correct' differences. You only have two copies of the data, so there is no way to determine which one is correct other than flipping a coin.
IF the RAID system keeps a checksum on each disk, I am wrong. Still, only zfs does that type of double-checking as far as I know (possibly btrfs or other newer FSes). RAID1 is for drive failure, not for error-detection or correction.
What this means to you is:
At best the system can only detect that there is a desynchronization and tell you about it. At worst, it will detect that, determine it is a problem, and decide which version is right without telling you.
If this resync happens quickly, you can know that it doesn't re-read the drive.
Reading and writing to hard disks shouldn't be 'stress', for the most part. I'd guess that setting the drive down, dropping it accidentally and being in a car on a bumpy road would be more stressful for a portable drive.