Question : Problem: NTBackup Error

NTBackup is giving me errors saying "...is not a valid drive, or you do not have access." This is happening in two different scenerios for me.  Two complete different servers and ntbackup programs and backing up to different media.  The strange part is that one night the backup works but the next night backup i get the error.  Nothing is changed in the time between.  

The backup i am most concerend about is my exchange mailbox store.  For example, last two nights ago the backup ran perfect.  Last night, 3 hours, 6 minutes, and 7 seconds in to my backup of server\Microsoft Information Store\First Storage Group is not a valid drive, or you do not have access.  

How can this be?  I am on the server as the admin, with access to everything--one night it works, and the next it doesnt.  Also, if i don't have access how was it able to start and backup 40gb in the 3 hours+?

Like i said, i have this same kind of issue on another server with mapped drives--but right now i just need to focus on this one.  Maybe the answer to this will also help me fix my other backups.  

Any ideas? I hope i provided enough info--if not let me know what you need.  Thanks for the help in advance!!

Answer : Problem: NTBackup Error

Yeah, I'd try that and see if it helps.  NTBACKUP may be writing temp files that are adding to the disk space issue so it works then it fails after a successful backup.