Not possible to encrypt Outlook pst file message

When I attempt to add an outlook pst file I get an error stating “It is not possible to encrypt th eselect file. This may be due to a restriction of teh USB flash drive file system ot the drive is full.” 

Anyone else run into a similar problem? 

SanDisk SecureAccess v3 6.0.34W

I created an archive of my email files (.pst files) and use that for backup. It won’t be possible to encrypt the actual live .pst file as then Outlook won’t be able to access and use it for loading your email folders.