External Drive Storage Problem or Glitch? (Mac)

I don't know if anyone is familiar with this issue but, I have a question... I'm on a Mac and I have an external drive with a capacity of 1 TB. Earlier, I deleted a lot of unneeded data off of it, freeing up many gigabytes. After this, I manually calculated the total storage used on my external drive (folder by folder), which is 312.12 GB and another 27.6 MB. Strangely enough, when I go to About This Mac > Storage, it says I only have 260.73 GB available, yet I deleted a whopping 440 GB or so. Obviously there is still something causing the true available storage data to not show. How can I fix this?
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