I just got two 500 GB Crucial MX500 SSDs. When I check SMART one drive says Unused Reserved Block Count is 44 while the other drive is 37. Both are new drives and shows only 1 power cycle. I go to Crucial's website and of course they confused me even more:
"Attribute 180: Unused Reserved Block Count (Available Spare Blocks on PCIe SSDs)
Again, as the name implies, this is the count of extra blocks available to be used in case bad blocks need to be retired.
This number varies based on the underlying NAND architecture, the firmware architecture, and the user capacity of the drive,
but it usually starts in the thousands.
This number decreases as the number of retired blocks increases. When Attribute 180 reaches 0, the firmware will place
the SSD in read-only mode. The SSD will not be usable as a normal drive, but the user should be able to retrieve stored
data and transfer to a new device. It is strongly advised that if this number should get below 100 or so, the drive should
be replaced."
"Attribute 180: Unused Reserved Block Count (Available Spare Blocks on PCIe SSDs)
Again, as the name implies, this is the count of extra blocks available to be used in case bad blocks need to be retired.
This number varies based on the underlying NAND architecture, the firmware architecture, and the user capacity of the drive,
but it usually starts in the thousands.
This number decreases as the number of retired blocks increases. When Attribute 180 reaches 0, the firmware will place
the SSD in read-only mode. The SSD will not be usable as a normal drive, but the user should be able to retrieve stored
data and transfer to a new device. It is strongly advised that if this number should get below 100 or so, the drive should
be replaced."
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