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I use Windows. A year ago, I backuped my D: disk (500 GB) with Macrium Reflect (it stores file in its own format mrimg, similarly to DriveImageXML stores .xml) to external HDD. I don't remember if i've chosen compression. year after, when i wanted to open that backup, it wouldn't work, because some sector on that HDD (behind that image) was corrupted. For that reason, WHOLE BACKUP FILE become unreadable.

I don't want it to ever happen again. Is there any alternative to COPY-PASTE (Seems best way, but it needs about 5 hours) to make a backup of whole D disk , so as even if any sector will ever become bad on that external hdd, i could still access all available data in backup, and it won't make the image unreadable? Seems imaged-backup (which results in 1 file) is unsafe, in case it becomes unreadable file?

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Monitoring the hard drive is the best option.

You can start with SMART monitoring. SMART stands for Self Monitoring and Repairing Tool. Particularly disks with high ECC (error correcting codes) should be replaced because those weak sectors will eventually fail.

SMART Attributes Data Structure revision number: 10
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME          FLAG     VALUE WORST THRESH TYPE      UPDATED  WHEN_FAILED RAW_VALUE
  1 Raw_Read_Error_Rate     0x000f   114   100   006    Pre-fail  Always       -       61609160
  3 Spin_Up_Time            0x0003   093   092   000    Pre-fail  Always       -       0
  4 Start_Stop_Count        0x0032   100   100   020    Old_age   Always       -       195
  5 Reallocated_Sector_Ct   0x0033   100   100   010    Pre-fail  Always       -       0
  7 Seek_Error_Rate         0x000f   085   060   030    Pre-fail  Always       -       4648073590
  9 Power_On_Hours          0x0032   077   077   000    Old_age   Always       -       20551
 10 Spin_Retry_Count        0x0013   100   100   097    Pre-fail  Always       -       0
 12 Power_Cycle_Count       0x0032   100   100   020    Old_age   Always       -       32
183 Runtime_Bad_Block       0x0032   100   100   000    Old_age   Always       -       0
184 End-to-End_Error        0x0032   100   100   099    Old_age   Always       -       0
187 Reported_Uncorrect      0x0032   100   100   000    Old_age   Always       -       0
188 Command_Timeout         0x0032   100   099   000    Old_age   Always       -       6 6 12
189 High_Fly_Writes         0x003a   061   061   000    Old_age   Always       -       39
190 Airflow_Temperature_Cel 0x0022   061   045   045    Old_age   Always   In_the_past 39 (Min/Max 33/55)
191 G-Sense_Error_Rate      0x0032   100   100   000    Old_age   Always       -       0
192 Power-Off_Retract_Count 0x0032   100   100   000    Old_age   Always       -       0
193 Load_Cycle_Count        0x0032   039   039   000    Old_age   Always       -       122569
194 Temperature_Celsius     0x0022   039   055   000    Old_age   Always       -       39 (0 21 0 0 0)
195 Hardware_ECC_Recovered  0x001a   114   100   000    Old_age   Always       -       61609160
197 Current_Pending_Sector  0x0012   100   100   000    Old_age   Always       -       0
198 Offline_Uncorrectable   0x0010   100   100   000    Old_age   Offline      -       0
199 UDMA_CRC_Error_Count    0x003e   200   200   000    Old_age   Always       -       0
240 Head_Flying_Hours       0x0000   100   253   000    Old_age   Offline      -       9421h+55m+42.115s
241 Total_LBAs_Written      0x0000   100   253   000    Old_age   Offline      -       36542577472
242 Total_LBAs_Read         0x0000   100   253   000    Old_age   Offline      -       2583422390857

Note here: 195 Hardware_ECC_Recovered, here the drive read a sector the ECC data told it the sector was wrong and it recovered it. Also note the Raw_read_Error_Rate and Seek_Error_Rate. It's generally not important how many you have, but how fast the number increases. However, on bad drives these can easily reach into the millions or even billions. If your drive gets that high, replace it. Every time it has to do an ECC recovery the drive read speed slows down, and when you have millions the drive will really start to lag.

Quoting from Hard Drive ECC Errors! - Memofix's Data Recovery Blog:

When a hard drive reads a sector of data, it also reads a 50 byte ECC code that resides just after the real data. When the data was 1st written to the sector it ran a sophisticated algorithm on the 512 bytes of sector data and this resulted in a unique ECC code which can only be replicated by reading the exact same data. When a sector is read later, the drive attempts to validate the data by running the same algorithm on the data and comparing it to the previously stored ECC code. If the code doesn’t match, the disk drive produces an error code and prevents the transfer of the data. The hard drive will normally attempt to re-read the data up to 10 times as it tries to match the ECC code and this process slows the drive down considerably.

The ECC is a complex mathematical formula that can detect and correct bad sectors.

Refreshing the surface refers to reading the drive in small blocks of sectors and rewriting those blocks with the same contents, verifying they are all ok. Some software adds additional tricks like inverting all the data, writing to the drive re-reading and then inverting it again, to ensure all the sectors are in working order.

Every couple months refreshing the surface with something like SpinRite from grc.com.

A program called Victoria from http://hdd.by will give you timing for the surface of the disk.  The more sectors that take longer to read, the worst off it is.

Victoria
Above: You see clear signs that the drive is aging because there are so many green blocks; a brand new drive would almost entirely be in the lightest gray region with a few 100 in the 100 region (medium gray).

Finally, Parity files. http://www.quickpar.org.uk/.  This will generate repair files that will allow you to repair bad sectors in a given file.  The higher the percentage of parity data, the worse the error it can repair.  However, it shouldn't require much data to handle a few 100 bad sectors.  I don't know the exact ratio, but someone else here probably does.

options screen

Here you add the files you want to protect, set the protection level (2% should be enough for these purposes), and click create.
main program

Here's the resulting PAR files totaling about 75 MB:
resulting files

Here I intentional destroyed 10000 sectors (count=10000)  bs is block size of 512.

dd conv=notrunc if=/dev/zero of="Windows 10 64  16299.15.iso" bs=512 count=10000 seek=1

A mere 2% PAR data was able to recover at least 10,000 bad block 512 bytes per block.

What is parity data?

While this article covers it briefly:

https://www.dataclinic.co.uk/raid-parity-xor/

It is also a formula that very smart people developed to correct errors. When the parity program is run it generates these extra recovery bits.  It's based on XOR.

I have included some screen captures of QuickPAR, but WinRAR also has this ability. See here:
WinRAR – Add file to archive
Under "Options" you can set the percentage of recovery data; 2% should be enough to protect against bad sectors.  If you have 10,000 bad sectors, you need to replace the hard drive ASAP.

RAID 5/6 have to protect against one or two whole disks failing, so this percentage is orders of magnitude higher.

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    It would have been nice to explain that in more readable for non-tech persons: 1) what's SMART and why that matters 2)what is ECC 3)what does "refreshing the surface" by software do (and if it has free alernative as Spinrite is pricey) 4) what is/does parity do, or how to use them? – T.Todua Sep 3 at 21:10
  • @T.Todua Does that help? – cybernard Sep 3 at 23:38
  • wow, many thanks!! – T.Todua Sep 4 at 13:23
  • I've suggested an edit to the answer to make a bit more understandable (step by step) - superuser.com/review/suggested-edits/911070 but moderators rejected the modification. If you think the suggested edit is good, would be nice to confirm, as it's exactly how I (and probably other newbies in this field) would like to have an answer step-by-step... many thanks again. – T.Todua Sep 5 at 6:38
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    @wrecclesham please post your comments as one answer and i'll upvote. – T.Todua Sep 9 at 19:09
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Nothing can protect against disk corruption. The only protections I can think of are:

  • Format the target disk with slow (not quick) format to refresh all sectors
  • Verify the SMART data of the target disk for weakness
  • Take more than one backup
  • ...more than one backup, with at least one off-site in case the house burns down. – Tetsujin Sep 3 at 13:41
  • Actually this is NOT true. You can generate a PAR2 file set for any file that acts like RAID. You copy the whole file to a new hdd, and use the PAR2 file to repair the damaged sectors. This is done on usenet all the time. Then just have a couple copies of these files. – cybernard Sep 3 at 13:47
  • True: you still want 3 copies of it though. – Tetsujin Sep 3 at 13:49
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    @cybernard: So, say, one needs a backup in two copies and PAR2 backup in two copies. Seems too complicated. – harrymc Sep 3 at 14:24
  • You make your backup image. Then PAR2 it, and then copy and paste the PAR2 file to another storage device. (hdd,cloud,sdd, or whatever). Its not that complex, but point is it works – cybernard Sep 3 at 14:50

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