Why is a WD 4TB hard drive refusing to be used as an internal boot disk in an iMac 27" 2011
The problem of undetected GUID partition tables arises when disks with 512B aligned partition tables are mounted to 4k-only enclosures (or 512B disks partitioned in 4k-only enclosures with a 4k aligned partition table are attached internally).

The 512B GUID partition table looks like this in blocks (in parenthesis the "respective" start Byte/size in Byte):

                0     (0)     1   (512)   PMBR
                1   (512)     1   (512)   Pri GPT header
                2  (1024)    32 (16384)   Pri GPT table
               34             6  
               40 (20480)   ...           Start of first partition 4k aligned


The 4k GUID partition table looks like this in 4k blocks:

               0     (0)      1  (4096)   PMBR
               1  (4096)      1  (4096)   Pri GPT header
               2  (8192)      4 (16384)   Pri GPT table
               6 (24576)    ...           Start of first partition


Either GPT table can hold up to 128 partition entries.

The GPT standard requires that the primary GPT header must be located at block 1.

A 512B disk partitioned in a 4k-only enclosure won't have the primary GPT header starting at Byte 512 but at Byte 4096. The GPT of this disk attached internally won't be detected properly because the primary header is expected at Byte 512.

Only the PMBR seems to be valid and the "Partition Map Scheme" is therefore detected as "Master Boot Record". 

The opposite is also true: the GPT of a 512B disk partitioned internally (with the primary header starting at Byte 512) won't be detected in the 4k-only enclosure because there the primary header is expected at Byte 4096.