6TB Internal HDD for Time Machine

Yes but note:

Format large disks

Partitioned drive, Logical Volume Group, needs to be reformatted

The bug is in the ERASE function in Disk Utility, 10.8.4 and later. If you can use PARTITION directly without ERASE, you will also avoid the Bug.

Macs support drives with 'advanced' format (4K blocks) since 10.4.4. No issues there.

For Internal sled mounting:

The screw-depth penetration for 'bottom-mounting screws adjacent to the platters' on some drives over 3TB have been shortened. This may mean that the sled screws bottom out before they fit snugly and begin to compress the washers on the sled. If this is the case, your drive will not sit flat on the sled, and will not mate with the backplane connector without additional adjustment.

OWC also now sells sleds that uses alternate mounting holes in the HD to avoid that problem.

You can shorten those screws, or put a washer under the heads to ensure they can be made snug in the allowed depth. Also, now OWC/Macsales now sells sleds that use the alternate mounting screw locations to avoid this problem. However, these sleds are only for the 2009 and later Ptos

That bug appeared in 10.8.4, and has not yet been fixed.

using a Disk Utility from any older Mac OS X, including the 10.6 DVD (if your Mac can boot from it) or any saved Installer DVD or thumb drive will work.

Another work-around is to move that drive to an external enclosure and Erase it there (provided your enclosure is modern enough to deal with drives over 2.2TB.