Answer by Pow-Ian for 8GB stick thinks it's a 4GB one

Put the device in your windows machine and do the following:

1. On the start menu go to Run
2. In the box enter 'compmgmt.msc'
3. In the managment console select from the tree on the left 'Disk Managment'
4. On the right in the top list of disks, identify your flash drive and click the drive letter
5. That drive should now be focused in the list below.
6. I am assuming you are going to see a primary partition and a bunch of logical partitions
7. Delete the partitions until there is only one.
8. Format the only partition left.

This works in 85% of the problems with these drives I have seen. Occasionally someone will format one to be a boot device or a live linux distribution and there are a pile of inactive partitions for swap space and such.

As was mentioned in the comments, there is also a finite amount of writes for flash devices, but unless this was used as a primary os drive for a few years, I highly doubt you reached the write life on it.

I rarely suggest third party tools, however I have used this partition wizard a few times to fix stubborn partition issues. It is fairly intuitive and easy to use.

Finally a warning: If you are not sure which drive you need to fix the partitions on, or you are at all confused about the drive letter, stop before you start. You can seriously wreck a machine if you mess with the partitions on the wrong disk.