Time Machine requires 4.51 TB after FileVaulting drive

OSX 10.6.8, iMac, external 3 TB user account drive, external 2 TB TM drive.

I just FileVaulted the external 3 TB drive on which my user account resides.

Now when I try to run TM to an empty 2TB drive, it gives me this error message:

------------------------------------------------------------------------

Time Machine could not complete the backup.

This backup is too large for the backup disk. The backup

requires 4.51 TB but only 2.00 TB are available.

------------------------------------------------------------------------

Note the df command output below. The external account drive now has 2 mount points.

The FileVault mount shows 5.85 TB --- on a 3 TB drive, go figure!

When I issue a du -s on my account, it comes back with only 573 GB used.

$ df -k

Filesystem 1024-blocks Used Available Capacity Mounted on

/dev/disk0s2 976426672 52170964 923999708 6% /

devfs 114 114 0 100% /dev

map -hosts 0 0 0 100% /net

map auto_home 0 0 0 100% /home

/dev/disk3s2 2929930672 694447552 2235483120 24% /Volumes/External

/dev/disk1s1 1952795648 832288 1951963360 1% /Volumes/TM_Drive

/dev/disk4s2 5859861216 3624410864 2235450352 62% /Volumes/External/Users/transini

Is there some way to reset TM or overcome this glitch?