One word:  Compression.

Native capacities are with no compression.  For example, your 12GB 
drive can hold up to 12GB of un-compressed data.  Normal hardware 
compression can get up to 2:1 compression, giving a total maximum of 

Reality check, I've been getting about 27GB onto a 20GB/40GB drive. 
This is typical.  You should be able to get 17-18GB on the drive on 

Warning, my rant follows:

What I don't get is that they have not improved the compression on 
tape drives all that much.  I mean, compression programs regularly 
get 4:1 to 24:1 compression depending on the data.  Heck, a JPEG 
image is typically 12:1 compression.

Yes, I do know that a compressed file usually cannot be compressed 
any further.  Throw a JPEG file into a ZIP or Stuffit program and you 
might gain 1% at best.  However, the majority of files on a computer 
are not compressed.

I once took a System Folder that was 220MB in size and compressed it 
using Stuffit down to 28MB.  That's almost 8:1 compression.  Yes, the 
process to some time to do, but maybe it should be an option.  I'd 
like to get another 10GB out of this tape drive as there are weeks 
when the tape runs out on Thursday night needing just another 1-2 MB 
to finish the week.  I stopped backing up my laptop on the regular 
backups just to make room.  I really hate added more tapes to a set. 
I only backup 11 machines on this.  If I had a choice of 4:1 
compression, and could expect a yield of little better than 2:1, 
everything would fit and I could add another machine to the backup.

>I'm new to this list and subscribed because I have a question I hope one of
>you will be able to answer for me.
>I'm using Retrospect 4.2 for Mac, backing up a total of three Mac Servers to
>an APS DAT drive. The tapes I use are Sony DDS3 125P which state a Native
>(I'm not sure what Native means) capacity of 12.0 GB.
>After looking at the log of how much data is being backed up, it added up to
>14.3 GB. How can this be? It seems that the tape shouldn't be able to handle
>it but I receive no errors.
>Any explanations? Suggestions?
>thank you,
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