I have loaded a simple web cache page to a USB key. how is it possible for this USB key to be read by the 751G-2HnD. I have tried various options but I always have to format the USB key in the 751G-2HnD but then the usb key cannot be read by my PC unless I format it on the PC. I have tried the same with an SD card on the Rb450G and I get the same result. what am i missing ?.

ROS can read only Ext filesystem, but windows can't. You need special softwre to access Ext filesystem from Windows.
Simpliest solution now is to format the USB key in RB and transfer files via FTP, winbox or SMB.

Probably is formatted as "linux" filesystem like ext2 or ext3
put this usb key on a linux computer
and use fdisk to understand what type of filesystem is on it

RouterOS doesn't allow you to run a web server, but to simply exchange files in your network, you can use SMB which will be readable by any OS:
http://wiki.mikrotik.com/wiki/Manual:IP/SMB

Thank You. I'll try SMB. But essentially there is no way to write to the Mikrotik formated USB key externally unless its a Linux OS, is this correct ? . I actually use a MAC, I believe Paragon software will allow Ext2, 3 ,4 to be read and written on a MAC. I will also give this a try.