Hello,
I have a 160GB SATA hard disk for which I have enabled UDMA-6 in my motherboard as the motherboard does not have native SATA support.
the hdparm -I /dev/sda shows that UDMA-6 exists but it sets the device to UDMA-5.I have included the output of hdparm command below---
hdparm -I /dev/sda

/dev/sda:

ATA device, with non-removable media
Model Number: ST3160211AS
Serial Number: 5PT0G98G
Firmware Revision: 3.AAE
Standards:
Supported: 7 6 5 4
Likely used: 7
Configuration:
Logical max current
cylinders 16383 16383
heads 16 16
sectors/track 63 63
--
CHS current addressable sectors: 16514064
LBA user addressable sectors: 268435455
LBA48 user addressable sectors: 312581808
device size with M = 1024*1024: 152627 MBytes
device size with M = 1000*1000: 160041 MBytes (160 GB)
Capabilities:
LBA, IORDY(can be disabled)
Queue depth: 32
Standby timer values: spec'd by Standard, no device specific minimum
R/W multiple sector transfer: Max = 16 Current = 16
Recommended acoustic management value: 208, current value: 0
DMA: mdma0 mdma1 mdma2 udma0 udma1 udma2 udma3 udma4 *udma5 udma6
Cycle time: min=120ns recommended=120ns
PIO: pio0 pio1 pio2 pio3 pio4
Cycle time: no flow control=120ns IORDY flow control=120ns
Commands/features:
Enabled Supported:
* SMART feature set
Security Mode feature set
* Power Management feature set
* Write cache
* Look-ahead
* Host Protected Area feature set
* WRITE_BUFFER command
* READ_BUFFER command
* DOWNLOAD_MICROCODE
SET_MAX security extension
* 48-bit Address feature set
* Device Configuration Overlay feature set
* Mandatory FLUSH_CACHE
* FLUSH_CACHE_EXT
* SMART error logging
* SMART self-test
* General Purpose Logging feature set
* SATA-I signaling speed (1.5Gb/s)
* Native Command Queueing (NCQ)
* Phy event counters
Device-initiated interface power management
* Software settings preservation
Security:
Master password revision code = 65534
supported
not enabled
not locked
not frozen
not expired: security count
not supported: enhanced erase
Checksum: correct

The command hdparm -X70 /dev/sda gives an error.How do I set my hard disk to UDMA-6.

DMA does not need to be set with SATA drives, see; http://www.linuxquestions.org/questi...d.php?t=458088

but then why does hdparm show the mode as UDMA-5 and how do I ensure that i get maximum data transfer speed from my hard disk.

why does hdparm show the mode as UDMA-5 and how do I ensure that i get maximum data transfer speed from my hard disk.
the command hdparm -tT /dev/sda gives these results------->

hdparm -tT /dev/sda

/dev/sda:
Timing cached reads: 958 MB in 2.00 seconds = 479.06 MB/sec
Timing buffered disk reads: 200 MB in 3.00 seconds = 66.64 MB/sec

are these values optimal for a SATA hard disk??

Your timings look good, the values depend on the system specs. The motherboard/system specs and hard drive specs influence the overall timings. For comparison reasons only the output from my laptop;