Running the tab in normal mode allows the 64gb card to be read. However, if you start in CWM or TWRP recovery, you are no longer in the "normal" mode and the tab does not recognize the 64gb card. In recovery mode, it is possible that the memory allocations are limited where as in normal mode, 64gb is OK.
In this case, for rooting, maybe you should load the root file on internal memory and not the memory card. I use 4gb cards and neither CWM or TWRP could work with them. I think the issue is in the recovery programs and not the Tab, save for memory limitations set by the base firmware.
Again, JMHO and I may be way out in left field here.