Initial Seed failure on large backup

We're trying to create an Initial Seed backup for ~550 GB of photos/documents. The only way for us to do this with our current set up is via USB(2.0) to an external drive - our server (a Dell T420) does not have enough free space to create the backup locally and copy it over.

I purchased a 1 TB WD "Passport" drive and tried initially to back up to it (I created a folder, so the destination directory was "G:\Acronis Backup"). The backup took 19 hours before failing with "THE ERROR CODES" (see below). My initial thought was that maybe the space character in the destination name was the problem, so I changed the directory to just "G:\Backup". Tried again, same failure. Working with Acronis support, I ran chkdsk on G:\ and tried backing up to the root of our existing backup drive (F:\). 16 hours, same failure. Again with Acronis, we tried setting cataloging to "Fast" and suppressing warning messages before going to the Passport's root (G:\). We had the same failure this morning. Today, I tried creating a much smaller Initial Seed backup plan (~3 GB) on the root of my Passport external (G:\) which worked just fine after 10 minutes.

When these backups fail, an appropriately-sized backup archive is created at the destination. However, when I try to validate these backups I get a similar error - something to the effect of "can not find this file:".

I have an open ticket with Acronis, but I'm hoping for alternative suggestions here. My first thought is that maybe this is a timeout issue of some sort? My next thought is that maybe I can split this initial seed into multiple smaller files and just submit them like that, but I assume this will make the datacenter's head spin. Is there an obvious way to add files to an existing archive (as in - create a successful 10gb backup, and then just increment with another 10gb every few hours until it's the whole seed) that I'm missing? Or can anyone suggest a better alternative to get this seed copied over and working?

Here are THE ERROR CODES - the path names change with destination and timestamp each time, but the codes are the same:

***************************************
Command 'Backing Up' has failed.
Additional info:
-----------------------
Error code: 61
Module: 309
LineInfo 4a8728dc8a1c950f
Fields: $module : service_process_vs_398774
Message: Command 'Backing Up' has failed.
-----------------------
Error code: 46
Module: 64
LineInfo: a1d3981537c6878e
Fields: path : file:G:\Acronis%20Backup\PathCases(Seeded)_2014_08_22_13_47_23_113F.TIB, $module : disk_bundle_vs_38774
Message: Failed to open the backup location.
-------------------------
*****************************************

Thanks in advance for any advice!