It's a setting on your BB. You can disable it totally
To disable mass storage mode complete the following steps:
On BB, open:
Options > Media Card or Memory
To disable Mass Storage Mode, scroll down to Auto Enable Mass Storage Mode when connected and change it to No.
Press the Menu key and click Save.
Enter your smartphone password and click Enter (if prompted).
For more info see this doc KB13564-How to enable or disable mass storage mode on the BlackBerry smartphone