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Copyright (C) 1987, the Disk Doctor.
First published in the Rochester (PC)^3 News:
Picture City PC Programming Club
PO BOX 20342
Rochester, NY 14602
The Disk Doctor may be contacted at this
address, or via CIS [73147,414].
This material may be reproduced for internal
use by other not-for-profit groups, provided
this copyright notice is included.
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11:20pm
I was graphing some spreadsheet results
when "K" ran up to me, all out of breath.
"Doc, you've got to help me! Before they
find out." he huffed and puffed.
"Who?" I calmly /FS~Replaced.
"The rest of the guys in my group." K.
peered over his shoulder. "I accidently
trashed our departmental database. Can
you help me?"
I pried the floppy out of his sweaty fist.
"Is this the original disk?"
"No. I made an exact copy using DISKCOPY,
like you always tell people. That way you
can always go back if the working disk
gets messed up even worse."
"You're a smart kid. How did you get
yourself in this kind of trouble?"
He hesitated, as the words choked up in
his throat. "I was my fault. I was in a
hurry this morning and I think I shut off
the power while the disk drive was still
saving the file. The file is there in the
DIRrectory, but when I try to read it in,
the program says 'Part of file missing'."
"The CHKDSK command will give us a clue of
what we have here." I had his disk in my
drive. "There's the problem. You have
lost-chains."
K. looked at the screen as if it was in a
foreign language. "What's that?"
"The data is probably written out there on
the disk, but DOS doesn't know how to find
the next sector."
"Well, uhmm, how does it normally find the
next sector?"
11:22pm
I invited K. to sit down. "At the
beginning of every disk, there is a File
Allocation Table (commonly known as the
F.A.T.). The FAT contains an entry for
every sector on the disk, marking it as
either:
1) free and available,
2) part of a file (gives the location of
the next sector in the chain),
3) the end-of-a-file, or
4) a bad sector (isolated from further
use). "
I continued, "If you get an end-of-file
marker in the middle of a chain, the
missing segment is called a 'lost chain'.
The remaining sectors are not marked
available, so they still take up space,
but DOS can't access them. A bad-sector
marker would have the same effect, but in
this case, CHKDSK didn't report any bad
sectors."
"But why does the database report a error?
Why doesn't it read in as much as it can?"
"I not all that familiar with the database
program you're using, but my guess is that
it looks for certain 'signature' bytes at
the end of the file to know it has a valid
data file. It can't find the signature
because the file got truncated."
Gulp. "So what do you do now?"
"I'll use CHKDSK command with the /F
option to capture the sectors in the lost
chain, so we append them to your file.
Just one word of warning, K., if you ever
do any work with CHKDSK. A lot of the
fixes are irreversible, and sometimes they
don't come out like you want."
11:32am
"Here's a case, right here. You should
always answer 'Y' to CHKDSK's lost-chains
question. It creates a a link to the lost
chain, so you can look at the segment and
decide whether to delete it. If you
answer 'N', the sectors are marked
available, and you'll never see them
again."
"Gee. How come you know so much, Doc?
Where did you go to school?"
"Ha. There's no school that teaches this
stuff." The doctor paused and stared out
the window. "Something happened a long
time ago... I swore it would never happen
again. Now I only use my knowledge to
help other people..."
The flashback ended, and my computer came
into focus. "I'd rather not talk about my
past. Let's just get you back in business
again." "Now, I can just concatenate these
2 files with the COPY command. There.
Let's try to run it."
11:47am
"It works. Doc, you're a life-saver.
There's a good chance I can get this disk
back to my office before the other guys
find out what happened. How can I ever
repay you?"
"If you learned something today, then I am
repaid."
WARNING: The feats described in this
report are performed by carefully trained
specialists. Children should not attempt
these stunts at home without proper
supervision.
