Hi,
Sorry for late answer..
I meant to copy file with the datastore's files manager. The one on the web interface.
The WD Blue disk is old but functionnal. No SMART error or anything.
I did installed GhettoVCB and made a script to backup all the VM.
With GhettoVCB it copy at a constant 96MB/s which is pretty good.
I think that when i do copy VM with the datastore's file manager, the system inflates VMDK files, hence the slow rate and long time copy.
Thanks again for your replies and have a nice day!