itunes forgets my library location

Hi!

Let's say, I have my iTunes set up to use "/Volumes/BigPassport/iTunes Media" folder as collection folder.

It worked fine, but last days iTunes keeps forgetting this. Not sure when it happens, but problem is in there: I buy stuff from time to time, and iTunes tells me "Can't download", I go to settings and see that library setting is empty string instead of "/Volumes/BigPassport/iTunes Media".

What could be the reason for that? What should I do?