I'm providing you with the following detailed instructions as they apply to the Casper disk-cloning program which you have inferred you will be using to clone (comprehensively backup) the contents of your system.
Please note these instructions refer to the commercial version of Casper - not the Trial Edition. While the Trial Edition can be used for straight disk-to-disk cloning operations, it does not have partition-to-partition cloning capability. Note that many disk-cloning programs also do not posess this capability.
1. After installing Casper you would boot to your system with the SSD boot drive, with the 1 TB HDD and the 2 TB external (USB) HDD both connected. There is no need to initialize, nor partition, nor format the 2 TB USBEHD - it can be left in its out-of-the-box "virgin" state.
2. Using Casper, you would begin the operation of cloning the contents of the SSD boot drive to the 2 TB USBEHD that's the intended destination drive . On one of the opening windows - "Select the source" - select (highlight) the SSD 256 GB boot drive and click Next.
3. The "Select the destination hard disk" window opens. Click on the listing of the 2 TB USBEHD that will serve as your destination drive, i.e., the recipient of the SSD clone, and click Next.
4. The "How would you like to use the additional space on Disk
?" will soon appear. Select the option "Let me specify how the additional space is to be allocated" and click Next.
5. The "Specify how you would like to use the space on Disk
" window appears. Here you can set the size of the partition on the destination drive (USBEHD) to serve as the recipient of the cloned contents of your 256 GB boot drive. Ensure that the C: partition has been highlighted (it will be the default) and then enter whatever partition size you want. Presumably you would select 256 GB as the partition size, however, you could choose another size should you desire to do so. Click Next and then "Perform the copy now".
6. After you've completed this disk-cloning operation, shut down the PC. If it's practical, temporarily disconnect the internal boot drive (your SSD) and boot to your Seagate USB external HDD. Casper has a unique ability to boot to the OS that's been cloned to a USBEHD particularly if the PC is a non-OEM machine. No guarantees - but worth a try to determine that the clone is indeed a bootable drive when connected as a USB external device. Note that many systems will not permit a boot to the OS from a USBEHD.
7. In any event, if the system does boot to the USBEHD, access Disk Management where you will see the cloned drive listed as C: residing in a 256 GB partition (assuming that's the partition-size you chose during the disk-cloning operation). Note the drive will bear the same name/info designation as the source disk. You can change this to whatever label you desire in the Disk Management Properties.
8. The remaining disk-space on the 2 TB USBEHD (asside from any Recovery/EFI partitions, if applicable) will be Unallocated - that's the disk-space you will be using as a recipient of the cloned contents of your current 1 TB secondary HDD.
9. This initial disk-cloning (partition-cloning) operation will probably proceed quite slowly, however, future cloning operations will proceed much more rapidly because of Casper's SmartClone capability. So you will have a decided incentive to utilize Casper fairly frequently to clone the contents of your system so as to maintain a reasonably up-to-date comprehensive backup of your ENTIRE system.
CLONING THE CONTENTS OF THE 1 TB HDD TO THE 2 TB USBEHD...
1. Boot to your SSD boot drive with the 1 TB HDD connected. It's best that the cloned USBEHD be powered-off or disconnected at this point.
2. After the system has booted to the OS, reconnect or power-on your USBEHD.
3. You'll use Casper to clone the contents of your 1 TB secondary HDD to the USBEHD's unallocated disk-space.
4. On the opening "Select the source" window, select the 1 TB HDD as your source drive by clicking the "+" button beside the 1 TB HDD listing and selecting the drive. Click Next.
5. On the "Select the destination" window that opens select the "Upartitioned space" on the USBEHD and click Next.
6. On the next window select the default option to "Make the new drive as large as possible" so that the entire Unallocated space on the USBEHD will be utilized to hold the cloned contents of the 1 TB HDD. Click Next.
7. Casper will copy the contents of the 1 TB HDD to the Unallocated disk-space of the USBEHD.
So as a result of the above cloning operations you will have a USBEHD that contains the complete contents of your system - your SSD boot drive and your 1 TB secondary HDD residing on two separate partitions of the USBEHD. Hopefully, the USBEHD will also have the added advantage of being a boot drive although (depending upon your system) this is not always possible.
FUTURE CLONING OPERATIONS...
By & large you can use the above instructions for the future cloning of the contents of your SSD boot drive and your 1 TB secondary HDD to the 2 TB USBEHD. Obviously this assumes the USBEHD will contain sufficient available disk-space to accommodate the total data contents of both source drives.
As I mentioned above, if you're using Win 10 as your OS in a non-OEM environment there's a very good chance you will have the luxury of a bootable USBEHD - always a desirable feature in our estimation. Virtually every non-OEM ("generic") Win 10 OS system in which we've employed the Casper disk-cloning program using a USBEHD as the destination drive, the USB external drive booted to the OS straightaway. And in many Win 7 & Win 8.1 systems as well. Not a given - but we've experienced a good deal of success in this area.
But the important thing is now you will have the kind of peace-of-mind that comes with knowing that you have at hand a COMPLETE COMPREHENSIVE BACKUP of your ENTIRE system including your SSD boot drive and your internally-connected 1 TB secondary HDD. So should your boot drive become defective or hopelessly corrupted you have the wherewithal to return your system to a bootable, completely functional one with little effor or time on your part. What could be better?