Windows 7 Can't perform a backup to my desktop computer

Apologies if this is elsewhere, but I've been searching Google for the past hour and am continuing to have issues, and can't seem to find the solution that works.

I just got two new harddrives for my birthday - one for my desktop, and one for my laptop. The desktop drive is 3 TB, which Windows won't let me use the entirety of...this is sort of a separate issue and not what brings me here, because I figure I can get around this by using another drive for the operating system and just use the 3 TB drive for storage. The drive I want to do this with is the 250 GB drive currently in my laptop, which will be replaced with the other new drive I got for it, 500 GB.

My plan is to make a backup of all the data on my 250 GB laptop drive to the 3 TB (backing up from my laptop to my desktop, connected by the homegroup). Then I'll put the 500 GB into my laptop, copy everything back to it, and use the 250 GB as the second drive in my desktop. *deep breath* it's tedious, but the best solution I came up with to make the migration possible.

However, Windows 7 backup will not let me back up to the 3 TB drive in my desktop. The computers are connected over the network, and I've adjusted the read/write permissions to my user folder. I'm the administrator on both machines, yet Windows 7 backup still keeps failing. I downloaded Comodo Backup to see if I would have any more luck, but am still getting an Access Denied error and I do not know why...Windows 7 backup doesn't seem to be saying access denied anymore (after giving myself Full Control of my user folder), but it's still failing, and I'm not sure why. I've tried both backing up my entire C drive, as well as seeing if it would work with only a select backup (I chose my User folder and Program Files individually), but it still fails...

Does anyone know why this might not be working? It just occurred to me as I was typing this that it could possibly be related to me not having verified my desktop's Windows 7 install yet (I'd have to call the robot, and this would be a pain because I plan to reformat the drive after I'm done my temporary backups/transfers so I can use all 3 TB of space and not just the 746 GB that Windows lets me install to...verifying it before formatting it anyway just seemed pointless). Could this be why Comodo is still giving me an Access Denied error? Though, it wouldn't explain why the Windows 7 backup is no longer saying that's the case.

I've never done anything with network shares before - I've never even done a backup of this scale before. Any time I've reinstalled my operating system either A) a harddrive died and was unrecoverable so I had to start from scratch or B) I would just manually transfer over some files I had saved to an external drive. I'm not sure what the best way is to do this - I just want to get the data from my laptop to the drive in my desktop, and I'm not sure why nothing is cooperating. If there's more information I could provide to help troubleshoot, please let me know. I'd hate to have to just manually drag and drop all my files into the shared libraries...