External drives disconnecting?

I have 2 G-Tech drives daisy chained into each other then to the FW800 port on my thunderbolt display which is connected to my Macbook Air. Sometimes when transferring large files they suddenly unmount and I get the warning that I should properly unmount the drives. It is not a connection issue as I have tried various Firewire cables. Nor is it a drive issue as I have seen it happen on both drives whether or not they are connected as a single drive or daisy chained and they work fine otherwise. This has probably happened about 5 times already in the last couple of months and it has already corrupted some jpegs making them show artifacts or showing as incomplete pictures when I try to view them. Luckly I always have a backup copy of all of it so I havent lost anything. Anyone know why this is happening? This is a serious issue and I am trying to fix it ASAP.