If we are talking about the automatic backup on shutdown, it is my understanding that you cannot change this location.  It will make a dated backup file in the backups directory under the directory where the catalogue resides.  Certainly, there is no interface I can see in the catalog prefs for changing this.

When exiting Lightroom the Backup catalog dialog box will popup. You can choose the location from there. It will remain the same untill you change it.

vinsolo wrote:

 

when i close lightroom i ALWAYS backup my catalog to an external hard drive,

 

lightroom seems to write the backup to my internal hard drive at odd times, perhaps 1 of every ten backups, despite the fact that i have always selected the same external drive for this purpose. i depend on my previous selection to indicate the correct path, but this seems to change by itself at odd times.

 

i often don't check the backup path shown in the shutdown dialog box (because i never change it). however the dialog box does indicate the path that was used, regardless of the disk to which it wrote the file.

 

The time on which back ups are made is set in catalog settings. There are several settings to chose from.

The location can be chosen at the time the back up is made and can be anywhere you like. If you don't select a location LR will default to the location last chosen.

In LR 2 the back up was performed on starting LR, in LR 3 (by popular requests, I voted against the change and preferred LR 2's behaviour but seemed to be in a minority on this, pity there isn't a choice as this always ensured the option of a very recent back up if anything goes wrong, very useful on catalogs used by more than one person where the other person may skip the back up on closure)) it is performed on closure.

Whilst you can never rule out some weird behaviour in a computers OS that makes a random change to a setting, the only reason I can think of for seeing a different location for your backups in LR other than the one set by yourself is that you have inadvertently perhaps created a new catalog and this will default to whatever the default backup location built in by Adobe (I assume My Pictures or whatever on windows)

Here's another thread: http://forums.adobe.com/message/3187733#3187733

Looks like there's something with the way Mac OS X deals with USB drives.

Do you backup to and USB or FireWire drive? eSATA?