One of the great drawbacks of the electro-mechanical disks is their propensity to develop bad sectors. And unfortunately, SSDs don’t escape this problem.
Bad sectors are a problem for storage devices simply because they can result in inaccessible or lost data. Moreover, bad sectors often have a happy knack of developing in the same areas of your disk where your most important data is stored.
Typical Symptoms of Bad Sectors on an SSD include:
- Your S-ATA or PCIe SSD (such as Samsung, Micron, SK Hynix etc) is causing your computer to intermittently freeze.
- In Window’s Event Viewer, you see evidence of “bad blocks” being reported.
- Your S-ATA, PCIe (NVMe) or USB (3.1 / USB-C) SSD is not being recognised by your computer
- You can see your SSD’s folders and files in Finder (MacOS) or Explorer (Windows) but cannot copy them to another medium.
- You’re receive an “access is denied” error message when you try to access your Micron SSD in Windows.
- In MacOS, you see error messages like “First Aid found corruption” after running in-built disk repair utilities. Or, you see messages like “The disk you inserted was not readable by this computer”
- You’ve tried running a data recovery program like EaseUs or Recuva but it keeps on freezing.
- Checkdisk (Chkdsk) freezes at a particular point.
So, why do bad sectors or bad blocks develop on SSDs?
Well, there are a number of reasons. First of all, like with HDDs, SSDs actually leave the factory with some factory-marked bad blocks. This is because the manufacturing process for NAND is not perfect. Imperfections in the NAND wafer, from which NAND dies are cut, are almost inevitable.
As the SSD gets used, grown bad blocks (sometimes known as runtime bad blocks) start to develop. These can occur for a number of reasons including:
Wear and Tear – The insulation layer of the tunnel oxide in NAND cells begins to degrade due to the Fowler-Nordheim tunnelling process which occurs during P/E (Program/Erase) cycles. Altough the wear levelling (WL) algorithms are designed to evenly distribute block usage across the volume, WL is not a perfect process. And don’t forget that some types of NAND have lower endurance than others. On one end of the spectrum, you have high-endurance SLC NAND (which is actually rarely used even in industrial-class SSDs) while at the other end you have QLC NAND which is considered low endurance NAND. Or, to put it into perspective, a 1TB TLC SSD would typically have an endurance rating of 1 DWPD (Data Writes Per Day) while a 1TB QLC SSD would typically have an endurance rating of just .1 DWPD.
- Trapped Charge – Sometimes after prolonged usage, electronic charges can get trapped in the nitride layer between the NAND cells. This makes the voltage threshold for program/read or erase operations too high resulting in unreadable or unerasable sectors. The trapped charge problem can also be caused by improper shutdowns of the host system or by power supply issues with the SSD.
- Prolonged Storage – If flash-based storage devices such as SSDs have been left powered-off for a while, they can lose charge. This retention loss can result in blocks becoming unreadable and being marked bad by the disk’s Status Register. These bad blocks are also added to the Bad Block Table. Some SSD manufacturers include “refresh” algorithms in their controllers which are designed to recharge cells when the device is connected.
- Disturb Failure – NAND cells can get “disturbed” when a bit is unintentionally programmed from a “1” to “0” or vice versa. This occurs when the voltage for cells-to-programmed creates an electric field which interferes with neighbouring cells.
- Bad blocks or bad sectors can become very problematic when they start to develop in the System Area of an SSD. This can result in unreadable firmware or unreadable boot initialisation code. The latter scenario can result in your SSD failing to be recognised by your computer.Bad blocks occurring in the user addressable area of the disk can be managed. Most SSDs have a Bad Block Management (BBM) feature which marks blocks as bad (unreadable). BBM then uses “good” cells from the reserved section of the disk to substitute for the bad ones.
Fixing Bad Sectors on SSDs
Over the years commercial products have been patented and developed to cure bad sectors using methods such as hysteresis. But most of these solutions never really resolved the bad sector problem. Just as with HDDs, there is no real way to fix bad sectors on an SSD. However, an experienced data recovery technician can work around bad sectors and try and recover as much as your data as possible using specialised equipment.
Examples of specialised data recovery equipment include:
Slow Sector Reading
Equipment which slow-reads of sectors. The read timeout parameters on a standard operating system are configured for healthy disks. Data recovery equipment allows the technician to read the disk using modified read timeout settings. This means that sectors which a standard operating system such as (MacOS, Windows or a Linux-based OS) would report as “unreadable” are actually readable by the equipment.
Smaller Sector Sizes
Equipment which uses variable sector sizes. For example, an Apple MacOS system will typically read disks in increments of 4096 bytes. Professional-level data recovery equipment allows the technician to read data in increments as low as 16 bytes. This sort of granularity, along with delayed-reads, allows for successful data recovery from bad sector areas.
Data recovery companies can use equipment which can change the voltage supply to an SSD. This means that an S-ATA or PCIe (NVMe) SSD which is unreadable to a standard computer can be successfully read.If the System Area of your SSD has become damaged due to bad sectors, a firmware emulator can be used by a data recovery company to substitute for the original. This can result in previously inaccessible data being made accessible again.
Data Recovery from a Micron 2300 SSD
Here at Drive Rescue we recently came across a prime example of how bad sectors can affect a disk. The Micron 2300 512GB M.2 disk was taken from a Dell laptop. In the BIOS, the system reported SMART predictive failure. The disk was being recognised by the BIOS but not by Windows Explorer. The disk used an M.2 form factor and used 96-layer TLC NAND coupled with an in-house Micron controller. Initial diagnostics reveal that several firmware modules could not be read. Therefore, we used a firmware emulator to substitute for the damaged controller. However, the disk was still reporting extensive bad blocks. We set our data recovery equipment to use a read-timeout of over 20,000 milliseconds. We also set the sector retry rate to 3. Moreover, we used a read block size of just 64 sectors. These parameters gave substantially healthier disk-reads. After almost 24 hours on our recovery bench, the results were very pleasing. The most important files for our project manager client were .XLSX. PDF and .MPP (MS Project). These were all successfully recovered. They only files which were not recovered were some .MOV files which the client could download again anyway. Case closed and our project manager could back to managing projects instead of the painful and time-consuming task of reconstructing files.
Drive Rescue, Dublin, Ireland offer a complete SSD data recovery service for failed Micron SSDs including models such as Micron C300, Micron C400, Micron 1100 256GB, Micron 1100 512GB, Micron 2210, Micron 2200s, Micron 2200v, Micron 2300 NVMe, Micron 5100 Pro M.2, Micron 5200, Micron 5300, Micron M550, Micron mtfdhba512qfd, Micron mtfddav256tbn and Micron mtfddak512tbn. We recover from Micron SSDs that are not being detected or not recognised by your computer. We also recover from Bitlockered Micron SSDs. Excellent success rates and fast service.