Datarecovery from overwritten tapes was Re: Reading DC100a tapes

Re: I know that a few data recovery companies have specialized DDS drives that can read past the (new) end of such an overwritten DDS tape, but they tend to be very expensive. The basic problem is that at the end of any "session" of writing to a DDS, a "logical EOT (end-of-tape)" marker is written to the tape, and the drive firmware won't let you read or seek past it. Here's a technique I've used successfully in the past ... be aware that it overwrites (loses) a bit more data :( Put the DDS tape in drive, write-enabled. Write a enough data to it to positition the tape just beyond the EOT that was written by the accidental overwrite. (Thus, new data has overwritten the EOT marker.) (You could first determine the number of kilobytes of data on the tape prior to the new EOT, and write that many plus a few more KB.) Now, unplug the power to the DDS drive, and plug it back in. The drive should power up as normal, rewinding the tape to the start. This leaves a "gray" area on the tape, at the end of your last record and the start/middle of the first remaining old data record. Attempting to read over this area may or may not result in an error! (That's important to remember!) Now... Method 1 Simply try reading the data you wrote, and then the rest of the data. If you get it with no errors, great, you're done! Otherwise... The rest depends upon the firmware of your drive ... most drives I've tried won't let you continue reading past an error (sigh). Method 2 (smart/nice firmware) Read the data you wrote, until you encounter an error. Try a few times to read, ignoring any error that comes in. When you successfully read a record, it's old user data...save it and continue with reading/saving until you hit EOT (end-of-tape). Method 3 (dumb firmware...every DDS drive I've tried) Do a "seek to EOF" to find the first end-of-file marker on the tape. This should skip over the gray area with no problem. (I assume you didn't write any EOFs in your new data.) Now, read and save the data until EOT. The drawback with method 3 is that some tapes are written with software that's brain-dead with respect to performance and data recovery. E.g., tar ... which writes no EOFs during its work. (A backup program that writes an EOF after every file will produce a DDS tape that's MUCH more recoverable than a standard tar tape.) Said differently, if the original tape was a tar tape, method 3 won't work for you. You could try skipping records instead of EOFs, but I have never had success in this. Good luck! Stan sieler at allegro.com -- Stan Sieler sieler at allegro.com www.allegro.com/sieler/wanted/index.html