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Backup Exec tape partially overwritten (a few Mb) - can I recover data from previous backup?

We have a Data Server running Win 2k Server, with Backup Exec v.8.6 backing up files stored locally on the server on a nightly basis (assuming someone remembers to swap tapes when I'm not around).  Our tape rotation system is very simple and we use 'overwrite media without prompting' to simplify unattended backups.  ('Bad idea' someone will probably tell me).

Having lost a number of important files from the data server (user error - not me I hasten to add!) I was attempting to restore from the most recent backup tape.  Unfortunately, I didn't realise a previously scheduled backup had not happened, and when I inserted the tape, the scheduled backup immediately started.  I cancelled the operation as soon as I realised, but around 40Mb of data had been written to the tape.

I have attempted to rebuild the catalog on the tape using the Device: Inventory: Catalog option, but this simply gives me the catalog of the new, aborted backup.  Is there any way of recovering files from the previous backup, which presumably still exist in some form?  The original backup was around 20Gb of data - the files we need are just a few Mb of Excel spreadsheets.


Avatar of CrazyOne
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Quite honestly I don't know if the following work with tape of not

Take look at these.


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You have got two problems to deal with, the first is that the catalog on the tape is basicly stuffed. This AFAIR is fairly simple to fix as a copy is stored on the server as well. The killer is if an eod was written to the tape. If it has then you are going to have to use a data recovery service to get anything after the eod on the tape. They use special tape drives that will ignore the eod and allow them to hoover off the remaining bytes and reconstruct the data. A standard drive will stop at the eod marker. You can find out if the eod was written using mtfcheck.exe from the windows 2000 resource kit. Further info can be found here:;en-us;256065
The vogon website also has an article which clearly explains the problem.

I've experimented a bit with a couple of different tape drives here and it's the drive itself not the software which stops at the eod. I had thought that using something like dd might have at least let you read the bits at a low level into a file and let you reconstruct it from that but no joy I'm afraid.

I'm afraid that I don't fully understand you update.

This is my understanding of how it works:

EOD End Of Data should always be written to mark the end of the session. It is the EOD filemark that is used to append to tape. When appending to tape the application will have the drive space to the end of data and from there write a new session header.

EOM End Of Media on the other hand is the physical end of the tape and the drive can not read past that. Which is why there is a special trailer on the tape which will signal and Early Warning to the drive so the drive can tell the application the EOM is coming up soon and so that a continuation block can be written and the backup continue on the next tape.  

What I don't understand is how is it possible to ever append to tape if the drive does not have the capacity to read past the EOD filemark.

I'm not entirely sure I grok this either. I thought that was how they worked as well.

At the moment all I can say is that I can't get it to work. What appears to happen is that, as you have said, the tape drive seeks to the end of data in order to append further data. It then erases the eod and writes a new header. It can then add data and later read it but what I can't get it to do is space past an end of data and then read from the tape. From looking at the code involved it would seem to be a limitation (feature) of the drive firmware, I don't fully understand it (I'm looking through the source for linux drivers) but writing or erasure of eod seems to be hardwired into the firmware, as does the assumption that reads will complete at eod. Writes can erase an eod and the creation of a new header then allows appending. (I think the assumption is that spacing PAST the eod is nonsensical cos then you may end up with two eod's on the one tape) What is neccesary here however is that we don't create a new header in order to recover the data. Otherwise we are back at the same situation with a catalog which doesn't allow the recovery of files beyond the fried data.
regarding the assumption about the logic of spacing paste eod on reads I'm glarking that this is the case in order to prevent chaos when using non rewinding devices. You seem to not be able to space past an eod on either read or write, what the write operation at eod's first operation glarks as is erasing the eod. The design logic for having this hardwired into the drive(again an assumption at the moment) is that it's not subject to vagaries of OS or backup software design. However if anybody can correct me on this, as I may be dissapearing down a blind alley (up my own a***<g>) I'd be happy to be proved wrong.