If a datastore is not visible anymore after reboot check the following: \\

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First check that the device is still present and connected to the ESXi:
<cli>
storage core device list</cli>
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If you can still see the device but not the volume check if volume is present but unmounted:
<cli>esxcli storage vmfs snapshot list</cli>
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If you can see volume in the snapshot list then you can use the following to mount it:
<cli>esxcli storage vmfs snapshot mount -l datastorename</cli>