Deleting files doesn't free space

Solution 1:

The ext3 and ext4 family of filesystems reserve a portion of the disk to keep it from becoming truly "100% full", for stability reasons, and so that the FS can store metadata in some of that space.

You can directly tweak how much of the space is reserved:

tune2fs -m 1 /dev/sdXX

replacing "1" with the percentage (0 to 100) of the disk to reserve, and "XX" with the device node and partition number of the device node, so in your case "XX" would be "b2".

Analogy: you have a full bus, and the only remaining seat is next to a very large person who occupies a seat and a half by himself. A person who is very insistent on getting a seat walks up and demands to sit there. Although most people would consider the second seat taken, this person is insistent. So the large person goes, "Whoa, OK!" and squishes in to allow them to sit. But as soon as 1 person gets off the bus and opens up another seat, even if the passenger next to the large person moves into a seat, most people still consider the bus to be 100% full, because nobody wants to sit next to the large person.

Source

You can also check this to see Reserved Blocks count…

dumpe2fs -h /dev/sdb2

To quote the very intelligent user who nailed the issue in the source above:

You will see "Available" go positive when "Used" is reduced to below 0.95*136236548 blocks = 129424720 blocks roughly.

(we have to adjust the "0.95" to the reserved % in your specific case, and the 136236548 blocks to the total block size of your device).