Knowledgebase: Miscellaneous
What is S.M.A.R.T.
Posted by - NA -, Last modified by on 03 February 2008 17:30
S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology) is a technology, developed by major hard disk manufacturers, included in most modern hard disks. This technology monitors and logs many events while the disk is running.

Here is an article from PC Magazine to describe what S.M.A.R.T. is,,1759,1879485,00.asp

Or you can refer to Wikipedia,,_Analysis,_and_Reporting_Technology

In general, we should pay attention to these two attribute, "Reallocated Sectors Count" and "Current Pending Sector Count".

Reallocated Sectors Count

Count of reallocated sectors. When the hard drive finds a read/write/verification error, it marks this sector as "reallocated" and transfers data to a special reserved area (spare area). This process is also known as remapping, and "reallocated" sectors are called remaps. This is why, on modern hard disks, "bad blocks" cannot be found while testing the surface – all bad blocks are hidden in reallocated sectors. However, as the number of reallocated sectors increases, the read/write speed tends to decrease. The raw value normally represents a count of the number of bad sectors that have been found and remapped. Thus, the higher the attribute value, the more sectors the drive has had to reallocate.

Current Pending Sector Count

Number of "unstable" sectors (waiting to be remapped). If the unstable sector is subsequently written or read successfully, this value is decreased and the sector is not remapped. Read errors on the sector will not remap the sector, it will only be remapped on a failed write attempt. This can be problematic to test because cached writes will not remap the sector, only direct I/O writes to the disk.

While these two values go abnormal, Thecus NAS will show "Warning" to notify the customer in the disk information page. Please also check the page "Why my disk shows warning even after bad block scan?"

However, even if the result of S.M.A.R.T. is healthy or the values are located in acceptable range; hard disk might be fault without any symptom. According to the research from Google (, after the first scan error, drives are 39 times more likely to fail within 60 days than drives without scan errors. We strongly recommend that the important information must always be backed up.

We strongly recommend using enterprise-class hard drives. In the hard drive manufacturer's web site, we also found that hard disk's manufacturers do not recommend using low power desktop models for RAID or NAS. For example: WD official Datasheet (

- Business Critical RAID Environments – WD Caviar Green Hard Drives are not recommended for and are not warranted for use in RAID environments utilizing Enterprise HBAs and/or expanders and in multi-bay chassis, as they are not designed for, nor tested in, these specific types of RAID applications. For all Business Critical RAID applications, please consider WD's Enterprise Hard Drives that are specifically designed with RAID-specific, time-limited error recovery (TLER), are tested extensively in 24x7 RAID applications, and include features like enhanced RAFF technology and thermal extended burn-in testing.

keyword: SMART harddisk failure warning
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