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Advanced tools for diagnosing and recovering hard disk problems. (This page is work in progress. Please refrain from editing until the first draft is complete and this note has been removed. Comments under Discussion are nevertheless welcome. And if you think there are plenty of other topics of higher priority than this then you're right - but I just need to do a brain dump of what I've recently learnt before I forget it all.)

Summary

The S.M.A.R.T. data returned by a hard disk can give a useful indication of its health but there are other tools that can be used which go well beyond this. WARNING: some of these tools are DANGEROUS and should only be used in a kill-or-cure situation or if you are certain that a full system backup is available.

ddrescue

ddrescue is a linux tool for copying disks sector by sector. As such, it resembles the venerable Unix and linux utility but that is as far as the resemblance goes. Unlike dd, it will persists if it gets a read or write failure, and keeps a log of the blocks it has successfully processed. This makes it possible to run it multiple times, attemting to recover as much data as possible. It may therefore be a useful first tool to try in recovering data from a failing disk.

It can be installed by the shell command

sudo apt-get install gddrescue

The man ddrescue command gives full details of options.

Spinrite

Spinrite is a sophisticated hard disk maintenance utility, which works much harder than most others in order to recover data from failing disks. This allows the disk to rewite the data to alternate spare sectors. In many cases, Spinrite can use the erroroneous results of many failed reads, together with error correction bits, to deduce what the data must have been.

Is is designed for use with conventional (spinning) hard disks and should not normally be used on solid state disks (SSDs) as it will cause wear to the memory cells. However, it can be used on Level 2, which doesn't cause wear and can force an SSD to reallocate marginal cells. As such, it can be used on an SSD which has become slow return an SSD to as-new performance.

Spinrite runs under MSDOS or FreeDOS and comes as a bootable CD image, but is quite easly installed on a bootable memory stick. It costs $89 for a personal licence but comes with a no quibble satisfaction or money back guarantee. You may find a Restarter at a Restart Party who has a copy.

A drawback of Spinrite is that it can take many hours to run to completion, especially if it has to work hard to recover data. However, a computer which fails to boot may be suffering from a bad sector in the first few hundred MB, which it may be able to repair in much less time.

A new version of Spinrite is expected during the second half of 2014 which should be much faster. It should also include enhancements to free it from the DOS dependence, allow it to be run on a MAC, and offer a suspend mode. This latter would allow it to be started during a Restart Party and then put into a low power state to give the owner time to take it home and plug it in to complete.

Spinrite has been around for many years, and the current version 6.0 was released 10 years ago. An Internet search may reveal criticisms of of it, some of it vituperous. Nevertheless, whether or not it will fix the disk you are faced with today, many unsolicited testimonials indicate that it remails a useful tool.

MHDD

MHDD is a low level diagnostic and maintenance tool that runs under MSDOS or FreeDOS. It's best run from a DOS bootable USB memory stick. If you have Spinrite on a memory stick, add this to it too.

Faced with a slow running computer, MHDD will show very clearly whether the problem is a failing hard disk, performing many retries in order to read data.

Note that MHDD will destroy data if not used with care.

MHDD is basically a user interface to the ATA command set and more. A key feature is that it accesses the disk direct rather than going through the BIOS and hence gets a more accurate and uncensored view. For example it can read the SMART data even if the BIOS hides it. The following is a very brief survival guide to the most useful functions.

Command line flags

\ENABLEPRIMARY

Normally MHDD disables access to the primary disk on the assumption that this may well be what DOD is running from. Use this command line flag when running from a bootable USB memory stick if it's the primary disk you need to access.

Commands

MHDD issues a prompt, to which you can type a range of commands, a few of which are listed here. (Commands are case-insensitive.)

HELP - Gives a list of commands and brief descriptions

MAN <command> - Gives a fuller description of the named command.

PORT - Issue this command first to get a list of disks, then select the one you want by number.

EID - Report extended ID information from the disk.

SMART ATT - Report values of SMART attributes. (The F8 key is a synonym for this command.) Pay special attention to:

• Read error rate
• Relocated sectors count
• Relocate event count
• Current pending sectors

See the Wikipedia S.M.A.R.T. article for further details, and also below.

SCAN - Scan the disk

The disk is scanned, giving a graphic display of its state, and with the option of remapping bad sectors.

Several options are offered in a pop-up menu, in particular:

• Start, End - Start and end sectors for the scan, defaulting to the entire disk.
• Remap - Attempt to remap bad sectors, provided they can be read correctly (with difficulty).
• Erase delays - Erase sectors which take a long time to read, whether or not correctly. This should cause them to be remapped, but the data is lost.

To make sense of the Remap and Erase delays options, the criteria for a drive to automatically remap (or relocate) are normally roughly as follows, though they may vary:

1. If a read is eventually successful after retries and/or error correction, then if the retries and/or error correction were below a certain threshold, do nothing.
2. If the retries and/or error correction were above that threshold, but the data was eventually recovered, remap the sector (rewrite the data to a spare sector and mark the original unreadable).
3. If the data couldn't be recovered, mark the sector as "unstable", increment the count of "pending" sectors and return an error to the host computer.
4. If the sector had already been marked unstable but is now read correctly, remap it and decrement the "pending sectors" count.
5. If a write occurs to a sector marked as "unstable", remap it by diverting the write to a spare sector and decrement the "pending sectors" count.

Blocks of 255 sectors are each represented n the graphic display by a single blob. A brighter greyscale or a coloured blob indicates a slow read, suggesting the disk had trouble reading a sector. An "x" indicates a sector was unreadable.

Log files

Log files of a session are created in a sub-folder LOG in text format. Rename the LOG directory after a session so that a new session on a different disk and on another occasion will create new log files.

External links

• External links as bullet points