My first post in this furum so please be gentle

I have a old 559pro+ that has served me good over the years. Everything works fine exept my iscsi target witch has important files in it. I think the problem started since the last upgrade to 4.2.6. In windows explorer it now just says "F:\ is not avaiable , "access denied" and looks like a currupted harddrive. I have had this mapped in windows for years without problem. I also now tried mapping it on another computer with the same result. Also tried restarting the nas several times. Also tried doing a LUN backup and restore, same thing.
Does anyone have any suggestions for troubleshooting this?
Regards
Mattias