What is CRC on hard disk drives ?:
During data transfer between hard disk & computer (for
example when a file is opened, or data is saved to the hard
disk), a technique known as CRC is used to ensure the non-corruption
of this data during transfer. If an error is detected, a CRC
error is generated and the data is re-transmitted.
CRC errors on hard drives:
Drives that produce CRC errors frequently have difficultly
reconciling the data received with the transmitted. This can
be for a variety or reasons such as poor cabling, hard disk
microprogram corruption, or electronic related problems
Data recovery from hard disks suffering
with CRC problems: The likelyhood of successful data
recovery is dependent on the cause of the problem and how
long the problem has been occuring. Recovery can be relatively
simple for electronic related issues, to complex if microcode
corruption is involved. Additionally,
there are several malicious programs that will cause Windows
to report CRC errors.
In all instances please contact us on
0871 977 2525 for advice.
CRC application on hard disk drives:
The most common method of checking for errors after data is
transmitted is by checking the parity bit of a byte or word
that is transmitted. The sign of the parity bit detected at
the receiver has to match that of the transmitter. If there
is a mismatch, an error is flagged. Likewise, Cyclic Redundancy
Check (CRC) codes can be generated for each data transmission,
and these should be identical at both the transmitter and
the receiver. Any differences imply an error has occurred
during transmission.
CRC codes in the simplest form are generated
by dividing the data to be transmitted by a predetermined
divisor (or generator polynomial) using modulo-2 division.
When dividing binary numbers, this can be evaluated using
subtraction. The difference between normal division and modulo-2
division in binary is that for modulo-2, XOR values are evaluated
for the minuend and subtrahend. When the division is evaluated
what results is a Quotient plus a Remainder. The remainder
is what constitutes the checksum. (The divisor or generator
polynomial used for hard disk drives is defined as 11021h
or x16 + x12 + x5 + 1)
A special feature of modulo-2 division is used
for the detection of errors. In hard disk drives, the data
sector is made up of 512 bytes. If this is extended by 2 bytes
of 0 lengths, the new sector is 514 bytes in size. A checksum
can be calculated for this 514 byte sector using modulo-2
and this will be 2 bytes in width. If the 2 zero width bytes
of the 514 sector are replaced by the checksum evaluated,
a method for detecting errors has been integrated into the
sector. This is because on calculating the checksum of this
new 514 byte sector, this will result in a remainder of 0.
If the remainder is not zero, it implies an error has occurred.
Therefore, when the device controller writes
data on to the platters, it includes 2 bytes for the CRC checksum
in each sector. On reading back the sectors, if the checksum
is not equal to 0, then an error has occurred.
CRC checksums are also used to detect any errors
during an Ultra-DMA data burst between the drive and the host
pc. Both the device and host start with a seed of 4ABAh. A
CRC checksum is then calculated for every transition during
the DMA burst using the current CRC function and the 16-bit
word being transmitted. On completion of the ultra-DMA burst,
the host pc shall transmit its final calculated CRC function
to the drive which then compares this to its own checksum.
If there is a mismatch, it sets both the CRC flag and the
ABRT flag in the device Error Register. NB. CRC checksums
are only evaluated during ultra-DMA data transfer.
