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I'm having a weird problem with the hard drive my MBP - OmniDiskSweeper and Finder report that the drive has 0kb remaining, however in OmniDiskSweeper, adding up the sizes (this is after letting it measure the drive completely) - it looks like the contents only add up to 38gb, while the drive has 100gb.

Why isn't it calcuating the size properly?

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I'm not familiar with the stuff you mention, but the data in the file isn't all, there is internal fragmentation (i.e., on average the last block of each file/directory is only half full) and there is the disk space outside of the file used to keep track of it.

Also, some little-known characteristic of most Unix filesystem is that they have a limited supply of inodes; if they run out, the filesystem is often reported as full even if there is space available. There are also disk quotas, if you fill up your allotment, the disk is effectively full to you, even if there is plenty of free space.

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