Maybe you won a thumb drive at a PMUG meeting. Maybe you copied some files to it. Now, how much memory is left? Maybe you know how, but I had to search for the answer. Looking at the SanDisk site http://kb.sandisk.com and searching through a lengthy list of topics brought no quick answer. I emailed to SanDisk and got a reply this morning. It was written for PC people, but here's the Mac way.
Insert the thumb drive into the computer slot, right click with your mouse and hold it down on the icon of the thumb drive that comes up. Click on Get Info from the choices in the list. (Or left click if that’s how you’ve set up your mouse.) Up comes the little info box with the Kind, Date created, Date modified, Format, Capacity, Available space, Amount of space used. It shows date last opened, the name, preview, and sharing and permissions.
Here Prez Art Gorski jumps in with info: "Regarding how much free space is available, this has worked in exactly the same way on Macs from the beginning of time. Select ANY volume (internal hard drive, external hard drive, flash drive, floppy, whatever) in the Finder and do a Get Info from the menu."