Hmm. Let’s try to get the basics of RAID down first, shall we? Please understand that:
RAID 1 = two hard drives of 4TB each, which would be COMBINED together into a SINGLE volume of 4TB ONLY. This is called mirroring, because your data that is stored on Drive 1 is also copied and stored IDENTICALLY on Drive 2. This is the default for your EX2, which you have purchased with two hard drives prepopulated in the unit. RAID 1 is considered safer for your data, because if supposing Drive 1 dies tomorrow, you still have Drive 2 to recover your data from.
RAID 0 = two hard drives of 4TB each, which would be COMBINED together into a SINGLE volume, but with 8TB of drive space. This is called striping, because your data is split across both drives, with Drive 1 containing half your data and Drive 2 containing half the other data. People use this mode only when they require SPEED in data transfer, since you now have two hard drives sharing the load of reading and writing your data. BUT… the biggest difference here is… you will put your data at RISK! Because, if Drive 1 dies tomorrow, you would be UNABLE to read anything from Drive 2! This mode should be used only… and only if… you have another set of BACKUPS elsewhere. Otherwise, do NOT switch to this mode.
SPAN = a useless way of storing data, because it is almost but not entirely similar to RAID 0, and a drive failure could possibly lead to partial recovery of your files, with the other half on the failed drive lost forever. Don’t bother switching to this mode, again, unless you have another set of backups.
JBOD = two hard drives of 4TB each, which would be managed SEPARATELY as two entirely different volumes. You’ll get the maximum 4TB of drive space for each drive (excluding whatever folders and apps that are created on Drive 1), altogether giving you a total of 8TB - exactly what you paid for. But again, should Drive 1 fail tomorrow, you will only lose everything on Drive 1, unless you keep backups elsewhere. Your data on Drive 2 will still be safe. In JBOD, you lose the RAID 1 redundancy, and you also potentially lose the RAID 0 speed (the maximum speed you can transfer data using your LAN cables would still max out around 100MBps anyway.) JBOD is basically just putting in two hard drives like you would into a normal desktop computer, without doing anything special. It’s like having a C: drive and a D: drive on your NAS. If you really need to maximise your TOTAL usable storage, or as you said: “I was hoping to use the two drives as separate backups”, then pick JBOD, not RAID 0.
Now, when you’ve first purchased the EX2, the blue box will show you the TOTAL space the drives could hold, but the USABLE space that you get for data storage is entirely up to you. If you choose RAID 0 for the entire setup, your total drive space is automatically HALVED, because you’re copying the same data to two hard drives for redundancy. And if you choose to set a partial RAID setup, like say, creating a 1TB volume in RAID 1, you have now used up a total of 2TB - and the leftoever 6TB can only be used in SPAN mode. This gives you a total of 7TB in usable hard drive space. So you see, you have some flexibility in determining how much disk space you really need, along with how much risk you’re willing to throw at your data, lol. if you’re unsure, stick to the default RAID 1 and its combined 4TB drive space, and forget about the 8TB original figure.
Now, back to your question. Your EX2 is currently setup in RAID 1, therefore, your USABLE space is now half of what you just purchased. The Dashboard will therefore show you 4TB (or 3.93TB if we wanna be accurate) as the usable space, and 3.58TB left after your first batch of data has been copied, as you’ve correctly noted. Since you are using RAID 1, your EX2 automatically copies two sets of the same 300GB of data to each of your drives, so you don’t have to worry about copying them to the second drive, or updating any changed or deleted files by hand. The two drives will both use the same amount of disk space automatically, so if you copy another batch of data and Drive 1 goes to 3TB, the same thing will happen in Drive 2 - and your Dashboard will now show a COMBINED 3TB leftoever space. Again, RAID 1 does all this automatically, so there is no need to to copy, modify or delete your data separately in Drive 2.
As for verifying whether the data is copied perfectly, you could always pull out Drive 2 (but only AFTER you’ve fully shutdown the EX2) and place it into a desktop computer with Linux installed (the filesystem is in EXT4, which Windows cannot read) - but I will not recommend that you do this. As long as you’re using the EX2 out of the box without reformatting or changing RAID modes, and copying your data correctly with the recommended apps, and there are no red lights or error messages to be seen, please continue using it and allow your EX2 to work its magic. Good luck!