Hi Niel thanks for the quick reply,
My 250GB capacity drive is relatively full (at about 245GB). Given what you mentioned above, I assume I must need to have room for 20GB of surplus space for each backup I make?
And therefore, would I need 265GB (to backup the 245GB drive) for the first time I clone the drive?
Have I got that right?
If you're going to make a bootable clone, you need that amount of free space. If you're going to create one or more compressed disk images, ensure that the drive has an amount of free space equal to the amount of data you'll be backing up; the extra 20GB is there to allow a booted OS to run smoothly.