Ext USB HDD, AEBS, Odd behavior

I have a dual band AEBS to which I've attached a WD My Book Studio II 1TB HDD using the USB port. This serves as my network drive on which is my iTunes, iPhoto, movies, and documents for each of 4 family member users of the 3 macs, so whatever computer someone is on they have their documents, can access a common iTunes and iPhoto library, etc.

Everything works great, I've added the HDD to each users "login" items to it mounts whenever a user logs in, and "most" of the time the drive shows up as a "blue" network drive.

The odd behavior is that every so often the "blue" drive icon goes away and a folder icon with red "-" sign shows up with the drive name on it. I can eject that folder, open the primary disk icon and double click the HDD on the AEBS icon and, voila, all returns to normal.

What gives? Is the drive "dismounting" in some way when inactive for a while? Other than this everything is working fine . . . this is a bit goofy it seems.

Edited to add: running 10.6.2 on two iMacs (one older "white" duo core and one new aluminum iMac (thanks, Santa)).

bump. anyone?

here's a screen shot of this folder that shows up related to my drive. sometimes when this happens, iTunes and/or iPhoto cannot locate their library files (which are on the external drive). I can't find anything about what this folder is and why it gets created. this happens when there is only one user logged on, and when multiple users are on.

Any thoughts?

To my knowledge that usually means the user has restricted access to it. Does it happen to all the users or one specifically?

all users from time to time. You can eject the folder and reattach the basic external drive and all is well for a while, then that folder shows up again. The permissions are "read and write" for "everyone" on the external drive.