IBM AVVA storage disc vol. 60 Gb was brought on information recovery. The disc was not defined, the engine was not accelerated, a full size problem, as usual.
After diagnostics and printed circuit repair I began logic analysis. At the moment I faced with unpleasant feature of IBM discs, a plenty of so-called bad blocks. Bad blocks are these sectors, at reference to which the store marks a mistake UNC - not corrected error of integrity infringement of the sector control sum. As lyrical digression, I would like to tell in brief how the disk logic sector is arranged from the logic point of view.
It is arranged simply - a data field and the service field protected by the control sum on which logic HDD defines correctness of information reading. Usual ATA 20h reading command reads a sector and gives out a data field. In case of discrepancy of field CRC the store gives out an error code instead of DRQ bit. 21h command allows to subtract such a sector with the help of "Read long" option, and then, having separated a sector service part of, it is possible to receive the read user sector part of 512 bytes. By the way, this command is not supported any more for modern disks, but such way still can be used for IBM discs.
And I faced a lot of UNC bad blocks on the client disk. And the most unpleasant thing was that many of them were located in the MFT area where all files were described with their size, location, quantity of fragments, etc. Dark green sectors on a screenshot stand for sectors read with mistakes.
Screenshot of MFT table part
Contents of such a sector differ from a normal sector, it is clearly seen visually.
Sector read with a mistake
Correctly read sector
In this case, after sector-by-sector copying of all readable files to the serviceable disk, it is necessary to do best for correct reading of critical sectors of the files accommodation table (repeated reading with the analysis, various algorithms, etc.). In case of failure the only way is to edit damaged fragments of MFT recordings manually. Usually it is a long and tiresome task as this problem cannot be done automatically and each sector needs analytical approach.