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Old September 4th 03, 07:05 PM
Rob Turk
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"Paul Rubin" wrote in message
...
"Rob Turk" writes:
I've heard that if you write two consecutive EOF marks on a DDS or 8mm
tape, that's interpreted as end-of-tape and there's absolutely no way
to read past it. It's not like the old days of 9 track tape.


One more example of half-truth. ....
Reading past such an area is normally prevented in firmware, as anything
behind the EOD marker is not considered part of the current tape

sequence
(remember, sequential data). However, most drives can be persuaded to

skip
such an erase area to the next valid filemark, and can continue to read

from
there on. Data recovery centers use this feature to get 'overwritten'

data
back. I've written software to even automate the recovery process on
EXB-8200 through 8505XL,


The subject has come up here on c.a.s. or comp.periphs.scsi a few
times in the past and there don't seem to be any such methods
available to ordinary users. There was some kludge posted about
starting a read and powering down the drive at some critical instant
and powering it up again, or something like that, which sometimes
confuses the drive enough to read the data. Anyway, yes, I shouldn't
have said "absolutely no way" since it has to be possible if you can
modify the firmware. However, if it requires firmware mods or the
resources of a data recovery center, that's close enough to impossible
for most users.


What you describe was for 4mm DAT drives, start a write and power down, so
no EOD marker would be recorded. Then spacing over the (now corrupted) erase
area would work.

For Exabyte drives, for 8200's no mods were needed. The firmware (32KB of
code on an 8051 CPU) did not require any changes to space to the next
filemark after EOD. Not enought smarts to detect EOD while spacing ;-) For
all other 85xx drives a standard EE-image implementing 'directory support'
allowed you to do the same thing. No tricks, no switching power, just using
the features of the drive.

Rob