Dell T3500, freezes on Widnows boot with USB drive connected

I have a Dell Precision T3500, BIOS version A17 (which is the latest). It's running Windows 7 SP1 Ultimate 64-bit with all updates.

I find that if I boot the system with a USB drive connected, Windows freezes during the splash screen. The only way to boot the system is with the drive disconnected. I can safely connect the drive once the Login screen is on.
The system is a file server, so it's on 24/7. The issue is a nuisance when Windows reboots after an update or due to a power glitch, because it can't complete the reboot unattended.

The drive is Toshiba Canvio 3TB hard drive with USB 3.0 interface. I had used a USB 3.0 PCI-e card and also connected to the built-in USB 2.0 ports with the same results.

This same drive had been connected to a another machine with the same Windows version and booted fine. The trouble started when I put the Dell into service.
The external drive is configured with one Primary partition, not marked active.

Any ideas? I had Googled this a while ago and vaguely recall that this was seen by others as well.

Thanks

Please post back a screenshot of Disk Management - Post a Screen Capture Image
with all drives connected.

Here is the Disk Management view of my drives. The DVD-R/W item did not fit; it's empty.
I have quite a few partitions mounted in directories rather than as drive letters, including the USB drive in question. That drive is labeled "TOSHIBA EXT".
I had used the same arrangement successfully with the computer prior to the Dell.

I notice there is no drive letter for the external. You can add Drive Letter - Add, Change, or Remove in Windows 7 - Windows 7 Forums. If you wish to continue mounting it as you described then I'd first test by removing that as a possibility for blocking boot, then do several reboots confirming again you've unmounted and removed it completely.

In some cases where an external has been made bootable intentionally or not, this can block boot. To exclude this as a possibility, you'd need to move the data off to wipe it of bootable code using Diskpart Clean Command.

I will try to mount it with a drive letter and see whether that changes anything.

This drive was never bootable. Previously, in trying to remedy this issue I had noticed that the partition was active and removed the Active flag. I guess I can change it from Primary to Logical and see whether that helps.

Wiping is out of the question. Moving 2.7 TB is not a practical exercise.

The drive could have inadvertently been made bootable while it was Active-flagged if Startup Repairs were run.

I have external drives that I don't remember making bootable which blocked Startup, and resolved the situation by wiping the drive of all code. This has also worked here for others several dozen times at least.

I tried converting the partition to Logical. Then Microsoft installed updates and rebooted - no good, system still froze.

As I wrote in the OP, I had used a USB3.0 card in the beginning and encountered the same issue. Today I decided that I would put the card back and search for drivers, maybe that would help (I was skeptic about the drive bootability being the factor, given that the drive previously worked with another computer and still did not work after I changed the physical partition to a logical partition).

The card came with NEC drivers 1.0.9 dated in 2009, and when I installed the card back in the computer these drivers were active.

Searching Google now for "Renesas USB 3.0 pci-e driver" (the NEC chipset is by Renesas, or maybe NEC was renamed Renesas; I am not sure) I found that there are version 2.x and version 3.x drivers available. I installed drivers 3.0.20.0 from PP2U (USB3.0 Host to PCIe Adapter) and when the installation was complete the system rebooted fine with the drive connected! The drive was recognized and works fine.

To complete the picture: running the driver's Setup program puts the drivers in C:\Program Files (x86)\Renesas Electronics. I needed to use the Device Manager to "update driver" and Search My Computer to that location. I got the warning that the drivers may not be for my device, but they installed and worked fine.
Once the USB 3.0 Host Controller driver was installed I heard the device discovery beeps and the USB 3.0 Hub became an unrecognized device. I installed its drivers from the same location.

Thanks to all who helped.