External Hard Drive properties

Hello,

I would be extremely grateful for some help with my external hard drives.

I have an (actually 2) external hard drive connected to our system. We are trying to transfer some pics from sandisk to hard drive, like we always do. We are getting the message that there is not enough space on the hard drive. We have not had the drive that long and I thought we still had quite a bit of space on the second one.
I seem to be unable to get the properties of the drive. They both show and are accessible in the network but not as drives on the computer. How can I find out how much space is left on the drive? I am running a Windows Vista Home Premium 64 bit Gateway computer. Network router is ASUS and external drives are both Buffalo.

Thank you so much for your help with this matter