"does it supports sd 64 ?"
If you mean if it supports micro SD card of 64GB, Sony says yes, but for some people it has created problems as you can see in the link below.
I personally use a 32 GB microSD so can't really comment on the performance of a 64 GB card.
http://talk.sonymobile.com/t5/Xperia-Tablet-Z/Does-Sony-Tablet-Z-WiFi-32GB-Support-64GB-SDXC-Class-1...
"can the tablet read files if i conect the it to a wd hardisk using a otg cable ?"
Yes it can read files from a WD hard disk using OTG, but be aware of 2 things. Some hard disks require higher current from the than what XTZ microUSB port provides, so then the HD has to be provided separate power. If the current drawn through the microUSB port/ OTG is not sufficient for the WD hard disk, then the hard disk will not work.
Oh, and using the search function on the forum will be helpful too !
