Q1:  Can't use usb Flash Drive on Mac 8.6 Operation System.
        Please access Apple website ( http://www.apple.com/support ) to download usb Mass Storage driver for Mac 8.6 OS. For Mac 9.0 above, it's plug and play by using Mac built-in usb driver.

Q2.  Can't write data into usb Flash Drive
        You must have overwritten the capacity limit of usb Flash Drive. Please right click your mouse on the "Removable Disk" Select "Properties" to check the available capacity you can use. You can delete some files in order to write in the new data.

Q3.  Can usb Flash Drive be used under DOS/Windows 95/NT? 
        No, DOS, Windows 95/NT don't support usb devices.
        So usb Flash Drive can't be operated in those operation systems.