LaCie and OWC have 4-5 bay hot-swappable tray drives that use Apple formatting of nearly flavour. DROBO make 4-12 drive assemblies but you have to use THEIR formatting and support durations - a problematic limitation.
[Xavier Paredes]"Is there such a thing as a USB 3.0 multi bay enclosure that allows you to mount/unmount individual HDDs WITHOUT having to unmount the other drives or power off the unit? As big bonus it should have tool-less trays OR the option to get extra trays. I don't want to be fiddling with any screws each time I have to swap out drives."
I'm using a CineRAID enclosure that might be just what you are looking for. It can be used as a JBOD or RAID, is tool-less, and hot swappable, with USB 3.0 and eSATA connections. Best thing it's nearly silent.
I'm really happy with it. I have it loaded with 4 x 3TB WD NAS RED's for a 12GB RAID 5 storage.