Cannot mount external drives

Hi all!

I have an 2010 imac i5 32GB RAM, running on 10.9.2.

All of a sudden my machine does not mount ANY extrenal hard drive. It recognises them in Disk Utility, but they appear greyed and cannot be mounted. Not even disk repair. Last week though the drives were mounting on the system without any problems.

Yesterday I have upgraded the OS to 10.9.2. (from 10.6.8). Still the same problem, the drives are recognisable in DU but cannot be mounted.

All of the disks I've tried are NTFS format (originally formatted on a pc). Funny thing is I have VMware Fusion on my mac runing a virtual Windows 7 pc. Windows do recognise the drives and mount them without problem, but not the mac.

Also note that I'm using Tuxera NFS to work with these drives with no problem so far, until now.

Also checked the cables, I'm connecting the drives though USB ports, they are ok.

Any suggestions?