Backup filesize has to be bigger than data backed up?

[ldg123]

This has always bothered me but now it is more than just a bother.

 

I have Express 6.5.342, OEM version, that came with a WD Ext HDD.

 

Whenever i backup, once every few months, i do a complete backup of my C: to an external Hd. I make a new backup set each time(backup set A, backup set B etc) and keep the last 2-3 full backups and delete old backups and backup sets.

 

When i create the backup set each time i do a backup it asks me "How much of this disk should be used". I enter a GB amt a few gbs above how much is on my C: drive. For example if I am using 10gb on my C: drive I'll enter 12gb. I also use the default file compression.

 

Now i have to enter a gb amt way above what i am backing up or else during the backup the program will ask for another storage device.

 

For example i just backed up 12.2GB, with the file compression it ended up actual being 10.8GB. But when i made the "How much of this disk should be used?" amount 15Gb it wanted another storage device after backing up about 9GB. I had to go with 20GB to only backup 12.2GB. I know it still only took up 10.8GB but if this Ext HD only had 15gb free it won't have backed up 10.8gb(compressed).

 

Why does this amount have to so much higher than the actual amount backup up???

 

I'm using WinXP.

 

Also is it true this version wont work on Win 7?

 

TIA!