Just got an iMac - it won't recognize and allow me to save to an existing flash drive

I just became a new mac owner (converting from PC). I have several Excel files that I keep on a flash drive that I need to continue to use via iWork Numbers, but wish to continue saving on the flash drive, not on my mac HD. When I try to save one of these files, it does not give me the option to save in the flash drive the file was pulled from (also, the only way I can open the files is to explore through the flash drive; I can't get to the flash drive via the prompt to open a file in Numbers). Under the "devices" heading, it does not list this flash drive (but it does list another, brand new flash drive that has yet to be formatted in any way).

So, is this a formatting issue? Will I only be able to treat this first flash drive as a read-only device? Is there a way I can format it to Mac without destroying the data that is on it currently? I will be wanting to move and edit these programs, among others, between my Mac, and some PC's via this flash drive; will that not be possible?

I just tried opening one of the files from the first flash drive in question, and was then able to save it to the second, new flashdrive; I was also able to close and reopen the saved document from the new drive. The iMac seems to recognize and interact with the new drive just fine, even though I've not formatted it in any way (Mac still lists both drives as being formatted as "MS-DOS (FAT)").

If I can't get any info on it, I'll likely just port all the info over to the new drive and then try reformatting the old one, then port the info back to the old one.

Usually I'd suspect that the old one was accidentally formatted as NTFS, which OS X only supports "Read Only".

New USB sticks are usually formatted from the factory, and then as FAT32. You'd want the old memory stick formatted as FAT32 as well, and this should be possible using Disk Utility in /Applications/Utilities.

Keep in mind that you need to backup all the files on the old memory stick first.

That's what I was thinking I would need to do. Shouldn't be too much of a problem, just a bit of a hassle is all. Thanks for the suggestions.

Interestingly, my wife's old flash drive is also formated as FAT16 (which is what my old one also is), and hers works just fine...