Hi Luc,
All the datastores are VMFS 5.54 on a HP EVA SAN, ESX version is 5.0.0 1117897.
Odd that it went fine from Thick to EagerZeroedThick but not the other way around Image may be NSFW.
Clik here to view.
(Fan of the book by the way Image may be NSFW.
Clik here to view.)