Windows 7 - Portable hard Drive - not automatically mounted and visable

I have two Identical hard drives [1.5T WD in external caddys with both USB2 & eSata connections]. Both have been in use since I built computer Dec10.
Now I plug them in and their not visable anymore? Their shown as a usb or scsi drive but no assigned drive letter or their "name".
Their showing in the Disk manager and I found I had to manually tell it to mount and assign a drive letter[then their useable]. I tried another removable drive that was USB2 & firewire and the same thing. USB2 ,eSata or firewire and their all not visable in explorer. Yet I can plug in a flash drive and I can immediatly see it once loaded.
Is there a switch in registry or somewhere thats controls this.
All these drives were immediatly available in file explorer previously? Now I am having to use the DM to manually mount and assign them to read/write to them.

Hello vinorhat, welcome to the forum

The only advice I can give you is this; Click on type Disk Management, Click on "Create and format hard disc partitions". right click on your external drive and "select Change drive letter and paths" click on Change and choose the drive letter that you want to use, click on OK, accept the warning, close all windows and you are done.

Try using a letter farther down the line to avoid conflicts with other USB devices