Question : Problem: Capacity of DAT DDS4 tape?

We are using a DAT DDS4 tape on one of our PCs here, and using Microsoft's backup software that comes with Windows XP to back up some directories.  The last few days we have been getting 'The requested media failed to mount.  The operation was aborted' in the logs, and the tape is ejected when I come in in the morning.  I figured that the tape might be full, but in looking at the bytes backed up, the amount is around 18 Gigabytes (the numers were between (17,886,126,884 and 17,889,272,433 for the 3 I just looked at).  The data is all uncompressable (PDF files, which are already pretty tightly packed), but I thought that DATDDS4 tapes could hold 20 Gigabytes?  I'm just wondering what is happening to the other 2 Gigabytes on the tape?  Is there a built in database on the tape that is taking up that extra storage (and not getting reported in Microsoft's backup status log)?  We only need to back up a few more days worth of data (we are planning on upgrading to a DLT or LTO solution in the near future), and would like to get as much data on these tapes as possible.

Answer : Problem: Capacity of DAT DDS4 tape?

To answer your question, the likely reason is simply media vs file size. MS has a tendency to report one, but store it differently (depending on block/page sizes of the hard disks vs the tape media). There will also be the catelogue on the tape for later recoveries, but this is not usually that big, unless you have a very large number of small files in your 18GB directories?

Barny