When i assign a letter to a hd it is immidiatly deassigned

Hello my friends,

I own a pc that is backed up with seagate replica 500 Gb hard disk.
Everything was working perfectly.

I also bought a seagate freeagent 3TB hd to have a second (no system) back up

As i can see now (it might happend after usb 3 driver installation, i am not sure)
the freeagent hd works perfect.
The replica hard disk can be seen at disk manager but everytime i try to assign a letter to it appears for 1 second at "my computer" and immediatly disapears and the disk managment also has no letter assign to that disk . .

Do you have any sugesstions ?

Operating System
MS Windows 7 Ultimate 64-bit SP1
CPU
Intel Core i7 950 @ 3.07GHz 39 °C
Bloomfield 45nm Technology
RAM
12.0GB Triple-Channel DDR3 @ 539MHz (8-8-8-20)
Motherboard
Gigabyte Technology Co., Ltd. X58A-UD7 (Socket 1366) 54 °C
Graphics
SyncMaster (1280x1024@60Hz)
W2261 (1920x1080@60Hz)
ATI Radeon HD 5900 Series
ATI Radeon HD 5900 Series
Hard Drives
313GB Seagate ST3320620AS ATA Device (SATA)
293GB Maxtor Maxtor 6L300R0 ATA Device (PATA) 44 °C
1465GB Seagate ST31500341AS ATA Device (SATA) 50 °C
313GB Seagate ST3320620AS ATA Device (SATA) 45 °C
1465GB Western Digital WDC WD15EARS-00MVWB0 ATA Device (SATA) 33 °C
313GB Seagate ST3320620AS ATA Device (SATA) 48 °C
Optical Drives
Optiarc DVD RW AD-7240S ATA Device
DTSoftBusCd00
Audio
Realtek High Definition Audio

Does it happen with any letter? Have you tried a different one?
Do you have any network mapped drive?

Hello pbcopter . .
it happens with all letters and i do not have any mapped drives . ..

Thank you

This may be a dumb question but have you restarted the system since loading the USB 3 drivers?

sure my friend . . . . i have restarted many times .. as far as i can understand the problem might be the seagate external hard disks . . . replica and goflex . ..