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We have some data on very old DDS (1,2 and 3) DAT tapes. I tried reading them with:

dd if=/dev/st0 of=file bs=32k

but get many partial blocks and sometimes IO errors. Have also tried with:

dd if=/dev/st0 of=file bs=32k conv=noerror

which does not stop at partial blocks but does sometimes give IO errors. Some of these tapes contain tar archives but most of them contain a bunch of FITS files. What would be the best way to rescue as much data as possible from these tapes?

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Send 'em to Kroll Ontrack (or another professional data recovery company). Your restore efforts are much more likely to further damage the tapes than doing anything productive.

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To some degree I feel bad not voting for the answer that provided technical information, but the risk of additional loss due to half (or 3/4 or 7/8) measures, and not knowing how valuable the data is that is being recovered make me want to err on the side of caution. – Slartibartfast Aug 9 '11 at 6:11
I don't think the data is important enough to justify sending it to such a company. I would prefer to just read the maximum amount and spend a couple of hours regenerating the few files missing. – Pim Schellart Aug 9 '11 at 11:15

Have a look at ddrescue, which is written for exact this purpose. Depending on the importance of the data, a professional rescue service might be able to do much better, at a (likely very high) price.

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ddrescue only works with block devices that allow non sequential access (am I right?). I thought it would not work with /dev/st0 which is a character device? If not I might give it a try. – Pim Schellart Aug 9 '11 at 11:13

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