You've mentioned that you're running 7 pro, in an office, with a server, so....
Is this computer a domain member?
The reason I ask is that, there are any number of policy settings that can restrict access to "removable devices" which, in a domain, can be applied to an individual computer, or user, or OU container containing that individual computer or user.
Even if not a domain member, there may be a local policy setting or even more than one that can do the same thing, for instance check the local group policy editor for a setting here.
Hold the windows logo key and strike the "r" key and type
gpedit.msc
and hit enter.
Navigate to here
User Configuration->Administrative Template->System->Removable Storage Access
Check the right hand pane for any entries that might be "enabled". On a typical installation they should all show as "Not configured". There may be more else where that I am not familiar with.
Additionally you mentioned that "there is no security tab"....... that is likely due more to the fact that the USB thumb drive is formatted with something other than NTFS format, likely fat32 and nothing to concern yourself with.