indicates 99% full but it is not

There's way too much information we don't know here that would allow us to give you a meaningful answer.

If I do your math backwards, that says you're working with a total drive size of 250 MB, is that correct?

What file format are you using on the drive? Different formats have different amounts of overhead that reduces the usable space on the drive, so it's important to know how it's formatted in order to tell if the overhead is excessive or not.

What operating system are you running on the computer the drive is attached to? There are tools available to check the integrity of drives, but they're all different for various operating systems. We can't tell you how to check the drive without knowing what you're running.

Other than the information we'd like to have above, in general there are a couple of reasons for your drive to appear full when it isn't. One case would be a corrupted file system where blocks of data are marked as used when they really aren't; when that happens you get the type of situation you're facing where the drive thinks the free space is much less than it should be when you take the difference between the space that's actually used versus the total size of the drive.

Another issue comes up when the drive is failing; blocks of data (sectors) on the disk that have read or write errors will be marked as bad blocks in the operating system and will no longer be available for use. If that was happening on your drive to the extent that almost a quarter of the drive has been marked as bad then your drive is not long for this world.

Anyway, if you can answer some or all of the questions I've posed, we can figure out what your next steps should be toward figuring out why the space appears less than it should be and what can be done to fix the situation.