In the TrueCrypt GUI, choose the partion of the external drive, then select "Mount without Pre-Boot authentication" from the Tools menu.
This will let you mount the volume as if it were an encrypted device without the boot loader (the difference just being a few offsets as to where TrueCrypt should attempt to decrypt the volume header from)
after some googling and a few own tests i concluded it's faulty drive. others have similar problem too with WD15EADS disks. prolonged test on properly aligned ext3 partition also showed performance degradation.
i've run in loop:
dd if=/dev/zero of=/mnt/1_5tb0/out bs=1MB count=45000
and got deteriorating performance:
45000000000 bytes (45 GB) copied, 652.667 s, 68.9 MB/s
45000000000 bytes (45 GB) copied, 648.647 s, 69.4 MB/s
45000000000 bytes (45 GB) copied, 645.147 s, 69.8 MB/s
45000000000 bytes (45 GB) copied, 655.122 s, 68.7 MB/s
45000000000 bytes (45 GB) copied, 644.662 s, 69.8 MB/s
45000000000 bytes (45 GB) copied, 645.12 s, 69.8 MB/s
45000000000 bytes (45 GB) copied, 648.025 s, 69.4 MB/s
45000000000 bytes (45 GB) copied, 650.528 s, 69.2 MB/s
45000000000 bytes (45 GB) copied, 1247.87 s, 36.1 MB/s
45000000000 bytes (45 GB) copied, 1601.76 s, 28.1 MB/s
45000000000 bytes (45 GB) copied, 1776.75 s, 25.3 MB/s
at some moment system almost halts with very high iowait + very little disk activity in iostat [eg 2-10 iops/sec ].
ps.: previous 1.5TB disk was removed, new one was installed - all problems are gone. so it was hardware issue.
Best Answer
Truecrypt doesn't currently support system encryption on Mac OS X. Here is the list of OSes that currently support system encryption:
http://www.truecrypt.org/docs/?s=sys-encryption-supported-os