TrueCrypt v7.1a on OS X refuses to mount larger hard drives (3TB +) that have been formatted with 4KB byte sectors instead of the "old standard" 512k byte sectors.
OS X itself has had support for 4KB drives since Snow Leopard was released in 2009, but TrueCrypt was never updated to include support for these type of drives.
Previously I had discovered "TrueCrypt-Lenlo" which has fixed the 4KB issue in TrueCrypt v7.1a on OS X and I would like to request that these changes be merged into a new release of VeraCrypt for OS X.
Please see the following links for more information about the pre-existing TrueCrypt modifications that allow for 4KB support.
Also please note that if you download the following file (linked from the above site) there is a .diff file that outlines what changes this developer made to the source code to allow for 4KB support.