|From the Dell site it appears that this is a laptop. So it may be there is a limit to the size of SD card it willl read.|
For example: Acer Aspire 4yrs old cannot read etc. cards over 512Meg. So to use or access cards over that size one must use a usb-adapter. Then the card can be read etc. completely.
If a system is a desktop... possibly the same applies? So one may need to update drivers, or - again - use a usb-adapter?
How big is the card you are wishing to use: and have you contacted Dell to find maximum size of card that your laptop can read directly?
"Ewen " perhaps a little charity...? I appreciate many here do not post adequate info etc. or a clear request etc. on many an occasion. But from this poster's name it's a reasonable guess that English is not his prime/first language? And in his statement is an implied question?