It doesn't permanently damage them, but it does really tick them off.
DLT writes in a linear serpentine fashion, where it starts on one track, 
writes to the end of the tape, moves down a bit, then starts writing again 
in the opposite direction.  A crude illustration below:

+------------------------------------------------------+
| START ---------------------------------------------> |
| <--------------------------------------------------- |
| ---------------------------------------------------> |
| END <----------------------------------------------- |
+------------------------------------------------------+

To keep track of which tape file and block is on which stripe, it puts a 
directory at each end of the tape every time it finishes writing.  That's 
why DLTs take so darn long to eject.

When you bulk erase them, it's got to read the whole tape, figure out 
what's where (which is really nothing), and then re-write those 
directories.  It takes about 2.5 hours when this happens.

BTW, the same thing can happen if you drive gets wonky. ;)

At 12:41 PM 8/23/2001 -0400, Labonte, L R (Leo), SITS wrote:
>CAUTION: You may do damage to your DLT tapes by bulk
>erasing.  We only bulk erase when we throw tapes away.

---
W. Curtis Preston
Principal Consultant for Storage Designs,   your storage experts
Voice: 760 710 7017                            Fax: 760 710 7019
Webmaster: http://www.backupcentral.com curtis AT backupcentral DOT com