Just because you can't see the partitions, or they are not mountable doesn't mean that you cant reformat the SD card ... Of course you'll be able to reuse your cards. You just need to format the entire SD; just use Windows disk management or whatever other tool is available on Windows ...
On nix you can just use dd. On Linux, Disks and gparted see all the partitions just fine. I bet that Disk Utility in OSX also wont have an issue seeing an formatting the SD cards (haven't checked tho - so just a guess). Its just Windows being crap
[edit] here is one utility I found at random. Here is some other guide with links.
On nix you can just use dd. On Linux, Disks and gparted see all the partitions just fine. I bet that Disk Utility in OSX also wont have an issue seeing an formatting the SD cards (haven't checked tho - so just a guess). Its just Windows being crap

[edit] here is one utility I found at random. Here is some other guide with links.