Neil.B.Ennis – Page 17 – Technical Musings

Easy backup

by Neil.B.Ennis August 16, 2005 No Comments Shortlink

I downloaded Image for Windows (IFW) from http://www.terabyteunlimited.com/imagew.html

It lets me backup my entire hard drive to DVD or CD. Plus it makes the DVD or CD bootable. That means if my hard drive ever crashes, I just boot from the DVD, and it will automatically restore everything.

Amazing. Easy to use. Only $US 27.

My only gripes:

1. It doesn’t write to DVD-RW so you can’t re-use DVD’s.
2. Someone stupid like me could accidentally boot from a DVD and wipeout everything on the hard drive.

Both those gripes are solvable – just use cheap DVD-R disks, and (easier still) don’t be stupid.

I’ll email their tech dept and let you know if they have a solution for the DVD-RW’s.

Addendum (17-Aug-05)

IFW can handle DVD+RW disks without any problems. DVD-RW’s need to be fully blanked or brand-new before they will work. So I think I’ll buy some +RW’s and see how I go! The article is here: http://terabyteunlimited.com/kb/article.php?id=215

Harrdware, Software

Crash recovery – a litany of errors

by Neil.B.Ennis August 11, 2005 1 Comment Shortlink

Here’s how NOT to recover from a faulty hard drive.

I turned on my laptop and got an error while booting about a faulty NTLDR.SYS file – or some name like that. The error said to insert the Windows CD and select option “R” to recover.

My Acer laptop didn’t come with a Windows CD, it came with a “Recovery CD”. So I inserted that CD, booted from it, and the damned thing started formatting my hard drive. Yes there was some vague warnings in broken English about backing up data, but I was expecting to see a Windows message, and got something totally different.

“Ah!!!!” I screamed as I saw what was happening, and turned my machine off in the middle of the format.

Now the disk drive was partially formatted, and had no directory info on it.

So I plugged in my trusty USB drive backup, and went about getting the machine ready to Ghost the backup drive back to the original drive.

Getting a laptop to boot from a CD and run Ghost is an effort in itself and took me about 2 hours to figure out. If you want to try it yourself, get Ghost to burn a bootable floppy that does the same thing, and use something like UBCD to convert the floppy to a CD.

So I ghosted the backup drive back to the laptop, booted the laptop and…. NOTHING. The USB drive was faulty, and the backup drive was unreadable.

“Ah!!!!” I screamed again in a Homer Simpsonesque panic.

I ran Spinrite on the USB drive in a vain attempt to fix it. But the back up drive was faulty (remember, Neil?) and so it just made things worse – and I lost the partition table on the backup drive.

So I threw the USB drive caddy out the window, and connected te Backup drive to a spare IDE port on my desktop computer. Still no joy.

So I tried a thing called “Diskpatch” which is supposed to fix partition tables. It said a few encouraging words, and did absolutely nothing.

Finally, I tried a thing caled “iRecover”. It was able to see stuff on the drive. Most of it was scrambled, but I used it to pump as much corrupted data from the drive as I could.

It has been running for about 7 days, and has almost finished pumping everything from the 60GB backup drive. about 75% of the stuff is rubbish. I was able to recover one or two snippets that were some use to me.

Bottom line – I lost about 6 months worth of work – but was able to scrounge a lot of it back from other sources.

I NEVER EVER want to go through this again. It’s bad for my health, my marriage and my business. Here’s what I will do in future:

1. Backup more regularly to multiple locations.
2. Use something like NTI to create a bootable backup DVD of my drive.
3. Get a new Laptop – the Acer Travelmate is rubbish.
4. Never try to run windows repair on a faulty drive until I’ve pumped it for all the information I can.
5. Never trust a USB drive. Especially one that runs hot.
6. Backup more regularly to multiple locations.
7. Buy high quality equipment

The other thing with Laptops – don’t move them around until you’ve put the machine in standby mode. If you do, the diskdrive is still spinning, and there’s a good chance you’ll stuff the drive up.

You will have one of your disks crash on you sometime in the next few years. It’s happened to me 4 times in the last 12 months on 2 different machines. Make sure you can recover from it. Do something about it today.

iRecover is available from http://www.diydatarecovery.nl

It works, but it is damned slow.

Mathematics Mathematics

Loan repayment calculations and high school algebra

by Neil.B.Ennis March 12, 2005 1 Comment Shortlink

FV = (PMT*(1+RATE*TYPE)*(1-(1+RATE)^NPER)/RATE) – PV*(1+RATE)^NPER

This is the formula for the “Future Value” (FV) of an investment assuming an initial investment (PV), constant repayments (PMT) and a constant interest rate (RATE) over a fixed term of months (NPER). When “TYPE” is zero, loan repayments are made in advance. When “TYPE” is 1, loan repayments are made in arrears.

You can algebraically rearrange the formula to give an expression that calculates PMT.

You can also rearrange the formula to give an expression that calculates PV.

Both of these just require simple high-school algebra.

But you CAN’T rearrange the formula to give an expresion that calculates RATE or NPER.

To work out these figures, you have to use an iterative method where you “plug in” a guess of what you think the rate or period should be, evaulate the result, and adjust your guess accordingly.

If you use some smart iterative methods, you should only have to loop about 5 times. The simplest way is given two guess / result pairs (x1, y1) and (x2, y2), and wanting to find the “x” which will result in “y”, your next best guess is to try x = y*(x1-x2)/(y1-y2).

In subsequent iterations, discard the (x,y) pair furthest from the desired result, and repeat the process with the new results.