HD showing 74.21GB when it is a 110GB Drive

Question

After using my MacPro for about 6 months I noticed the overall space on my HD decreasing. I am not talking about space available but overall capacity. Info is now telling me that my 110GB drive is 74.21GB.
I have run Disk Utility and everything was reporting fine until recently when I got:
Checking volume information.
Volume Header needs minor repair
The volume Macintosh HD needs to be repaired.
Error: The underlying task reported failure on exit
1 HFS volume checked
Volume needs repair
This was while varifying the disk. However this report is a recent thing and didn't happen prior to the space on my HD dissapearing.
The same is also happening to an external USB drive that I use. It is showing around the same capacity when I know it should show about 93GB for a 100GB drive when mounted.
I have run TechTool Deluxe and yet this shows no errors telling me everything is fine. Even though everything is quite obviously not
I am working in Iraq at the moment and do not have my installation disk to hand so can not run repairs from my install DVD.
Any help would be greatly appreciated as I am rapidly runnining out of space and can not even install a recently purchased copy of Final Cut Pro.
Thanks in advance to all those that spend the time helping on these forums, I have gathered quite a few answers from here but can not find anything on this issue.
Cheers
Rog

Answer 1

AFAIK there are no 2.5" SATA hard drives in the market being advertised with 110 GByte. Standard sizes are 60, 80, 100, 120, 160 and 200 GByte.
An effective size of 74 GByte implies the drive has an advertised size of 80 GByte. There's nothing wrong with that. Hard disc manufacturers don't calculate with a conversion factor of 1024 as everybody else is, but they calculate with 1000. That's where the difference between advertised disc size and actual disc size come from.
The same applies to your 100 GByte external drive.
100 GByte * 1000 = 100,000 MByte
100,000 MByte * 1000 = 100,000,000 KByte
100,000,000 KByte * 1000 = 100,000,000,000 Byte
100,000,000,000.000 Byte / 1024 = 97,656,250.000 KByte
97,656,250.000 KByte / 1024 = 95,367.432 MByte
95,367.432 MByte / 1024 = 93.132258 GByte
P.S. Forget what I just wrote. I haven't read your post very thoroughly. You obviously know all that.
MacBook 2.0 GHz/2.0 GByte ; MacPro 2.66 GHz/3.0 GByte ; iPod G5/30.0 GByte   Mac OS X (10.4.8)