Disk Information Shown in Disk Utility

Question

Hi MP Folks
My 2012 Mac Pro got 4 physical disks installed, 1 x SSD + 3 x HDD. All  bays are used. As you know, clicking the blue Info button at the top of the Disk Utility window may bring your the basic facts and extra information about the disks listed in the left pane.
I thought the information given there should be same kind of data in the same format, but on my Mac Pro only the first 15 items are the same, the remaining information varies from disk to disk, as shown below for the disks from Bay 1 to 4.
As you can see above, after S.M.A.R.T Status, the items are different. I got a few questions here.
1. Why does the APPLE HDD HDS723020ALA640 in Bay 2 have no further information while all others have more reliability data collected?
2. Why does the WDC WD10EAVS-00D7B0 in Bay 4 have a different Partition Map Scheme: "Apple Partition Map" instead of "GUID Partition Table"? I did use the same way that I did on other disks to partition and initialise this 1TB disk.
3. In stead of assigning Disk Identifier in the order of drive bay, why does "Bay 2" have "disk3" and "Bay 4" have"disk1"?
Thank heaps in advance.
Regards,
B

Answer 1

Data are recorded on the disk surface with a semi-redundant error-correcting code. Hardware and firmware in the drive are used to accomplish correction, if possible. Small bursts of errors are often correctable, but sometimes it takes many, many re-tries to obtain correctable data. Failure to obtain correctable data after many re-tries produces an I/O Error.
I/O Error tells you the data pattern in a particular block you are trying to read is very difficult to read [most likely], or possibly was written wrong or partially over-written with other data if the power failed while writing [extremely unlikely].  This is most often caused by subtle changes to the magnetic regions that make up the physical data, causing  a few bits to flip.
Getting an actual I/O Error is a desperate situation, much more extreme than the accumulation of minor data errors in the counter. There are data there, but they cannot be read. A drive with I/O Error has Bad Blocks, or may be failing completely. A drive with an accumulation of read errors means some blocks needed to be re-tried before correctable data were read.
The drive notes these problems, and maintains a "suspect list" of block numbers that are candidates for block substitution, and reports how many are on this list in its SMART status (accessible from Internally-mounted drives only). The drive is capable of substituting a spare block nearby, but cannot substitute another block unless and until new data are supplied, because doing so may poke a hole in the middle of a file.
Re-Writing the drive [traditionally with one pass of Zeroes] supplies that new data, and gives the drive an opportunity to check for proper data retention immediately after writing. Any blocks found not able to retain the new data have spares substituted, up to a maximum number of blocks in one pass (typically ten blocks).
Large scale studies performed by google in their servers indicate that drives that develop Bad Blocks in 24/7 use will typically need to be removed for too many Bad Blocks within 6 months.