Remaining Memory Card Space Discrepancy

So, I'm using SD2Vita, have a bunch of games installed via MaiDump, .vpk, and nonpdrm. Also, have a boatload of PSP .cso and ROM files for other systems. Rocking a 256GB Samsung microSD.

I keep getting the message notification on my Live Area that says "less than 40MBs remain on this card", or something to that effect. But when I connect the same microSD card to my computer, it says there are at least twenty GBs left on the card.

What gives?