Most of the time, you just pop the drives into their proper slots in the "toaster" and start the process, then come back later and it's finished but, if you use and love the convenience of dual-bay stand-alone cloners for archiving/backups, there are some times that stuff can go south so I share these tips/solutions:

(1) Don't blame anybody but yourself if you try to clone a drives with a greater capacity than the manufacturer's rating. If you're lucky, you'll just waste time; if unlucky, you may trigger a set of events with Windows10 that is regrettable (see CAUTION in #5, below).

(2) Paranoia pays off: Check and double check that Source and Target/Destination drives are in the right slots. Making the markings on the toaster legible or affixing labels is not a bad idea.

(3) I always disconnect the connections to the computer for standalone cloning. It's probably not necessary but when a large capacity drive doesn't appear to be cloning for the major part of a day, it's one less issue to have to worry about or consider (also, note #5, below).

(4) You usually have to make sure that the target/destination drive has been initialized and subsequently formatted before it can become a clone. You can do this with the Manage->Storage->Disk Management Snap-in feature of Windows10's This PC or with a piece of third-part software like EaseUS's Partition Magic (which has been a life-saver for me). After initialization, a quick format has never caused me any issues and is additional insurance that the cloning will complete.

(5) What happens if the power gets interrupted during a running cloning operation? Usually, just start over again. CAUTION: If your cables are connected to the computer at this point and your computer is on, turning on the toaster may cause a Kernal Error Blue Screen and an involuntary rebooting of your computer. If this happens two or three times in a row, you may have to go through the automatic Window Repair sequence (if you're lucky) or, if that fails, one of the Window's Advanced reset options---welcome to your nightmare. Don't turn on your computer and the toaster with the interrupted target drive in it until you have taken care of the target drive (usually, just restart the cloning process).

(6) What happens if YOU have to interrupt/stop a running cloning operation. First: Be aware of the indicator light sequence for your toaster. I have one that looks like it is approaching completion then, all of a sudden, looks like it's starting all over again---which can be scary after running for hours and hours with a large-capacity cloning operation (thankfully, the second cycle completes much faster). None of my toasters have a stop/cancel function so you basically have to stop the running operation by shutting it off. Re-read the CAUTION in #5, above. If you shut it off because the cloning is not working (the cloning operation has been running way too long, no progress indicated, etc.), read on. Try cloning a lower capacity drive to the corrupted Target drive to recover its functionality and to stop it from resetting Windows with a Kernal Error Blue Screen. The "smaller" the low-capacity drive, the quicker you'll get through this process. After the Target drive successfully clones anew, you can connect it, reformat it (quick format is ok), and expand it all the way. It should then be ready to repeat what you started out to do, your initial cloning operation.

(7) I have pair of 2TB drives, one Hitachi and one Seagate, that will not clone in one toaster (rated to 4TB) but will clone in another toaster (rated to 12TB). I have a pair of 8TB HGST drives, 10TB HGST drives, and 12TB Arsenal drives, same models, same capacities, purchased together in matching pairs, etc.---that will not clone each other even though all the required parameters are met. I do have a work-around that you can try for the issue of seemingly identical drives or identical capacity drives that will not clone: Use third-party partitioning software (I use EaseUS Partition Magic) to reduce/unallocate some space on the Source drive. In other words, make it appear "smaller" than its full raw capacity. Don't use the native Windows10 utilities for this because they will likely destroy all your data in the process. Third party software like Partition Magic will not destroy existing data, will not allow you to reduce the partition beyond the capacity needed for the data already on it, and will do so more rapidly and efficiently. Once you have reduced the apparent capacity of your Source drive, restart the cloning process. This has worked for me with the 8TB, 10TB and 12TB drives. After the cloning is completed I use the third-party partition software to expand the partitions to the full capacity of the drives.

(8) If your cloning was successful and you connected the toaster to the computer, you may notice that only one of the Source/Destination pair is appearing in the Explorer drives list. Since both drives now have identical signatures, they appear to be "colliding" to Windows and one of them was placed "offline." You can use Windows to place it "on line" and it is better to do this before you run you third-party partition "resizing" software since, even if it will see the off-line drive, it may not present you with the necessary options to resize/expand it:
Explorer->This PC->Manage->Storage->Disk Management; then find/scroll to the offline drive in the bottom frame and right-click on the blue "offline" circle and select "Online." After a moment the drive label and letter will appear and now it will be active in Explorer and ready to be processed with your third-party partition software, to expand the successfully cloned drives to full capacity.

This may seem like a lot of stuff but nothing here is as time-consuming as trying to transfer 8-10TB of files from one drive to another, either manually or via backup software.

Partition Magic is available on a trial basis to perform the functions mentioned here. I'm not affiliated except as a long-time satisfied customer. Others here might have other suggestions/recommendations. A primary consideration is that your data is preserved/not erased during re-sizing operations.

Later,
Paj
8^)