Do NOT try what you propose. "New to the RAID game" - OK, so we'll outline a few important things you don't understand yet.
There are many types of RAID, and all are different. The Particular unit you cite, a Promise Technology Pegasus2 R2+, comes able to use either RAID0 or RAID1 (not both simultaneously) using 2 HDD units. The default configuration supplied contains two 3TB HDD units set up as a RAID0 for a total capacity of 6 TB with relatively fast data access times. So I'm assuming that is what you have.
You should also be aware that there is no universal "standard" for RAID. Thus, the HDD units in your system can be used properly in your system, but you cannot expect to be able to move the HDD units to a different RAID enclosure and still be able to read any of their data. Not that you have any plan to do that - just to let you know.
In a RAID0 system, all data to be stored is split up into Sector-size pieces (a Sector usually is 512 bytes) so that, for each chunk of data, the first Sector is written to the first HDD unit, and the second to the other unit. It keeps alternating back and forth like that until all the data chunk has been written. Similarly, when you read back a file, it pulls the Sectors worth of data off the two drives in alternating fashion. This gives the RAID0 array system two important features which appeal to many: the total capacity of the system is the SUM of the capacities of the two HDD units used (in your case, 3TB + 3TB = 6 TB); and, data access is faster than a single HDD because while one HDD is busy writing (or reading) a Sector, the other HDD unit can already be hunting its heads for the correct Sector it will need next.
The major downside to RAID0, however, is exactly this layout factor. The data from every file is spread over TWO HDD units, and to use it BOTH of those units must be functioning correctly. If there is a data error on one Sector of one HDD, you have a corrupt file, very much like what would happen for a single drive. BUT if one of the HDD units fails completely in some manner, then NONE of your data is available at all! The remaining working HDD unit only has half of your data (every second Sector's worth). This means that, statistically, the probability of failure and complete data loss for the RAID0 system is twice as high as for a single drive. For that reason, users of RAID0 systems really should pay even more attention that others to the question of secure frequent data backups, in case such a disaster should occur.
In certain other types of RAID systems one can repair a failed RAID array by replacing one HDD unit that has failed. This can be done with RAID1 because that system uses two drives also, but writes identical information to each so that one drive is an exact and complete duplicate of the other. If one fails and is replaced, the system can use the remaining good HDD, which contains ALL of the data, to make another good copy of that data to the replacement HDD unit and then re-establish the RAID1 array. In RAID5 systems, the data itself plus some special calculated data are written over several HDD units. The design is such that, if one HDD unit fails and is replaced with a new blank one, the data remaining on the other HDD units can be re-processed to re-create the info that was lost on the failed drive, and that can then be restored to the replacement unit. Thus, a RAID5 system can recover all the data lost from the failed HDD unit and completely restore the RAID5 system. These are systems that can benefit from the "Hot Swap" feature - you can remove a failed HDD unit and replace it with a new one without shutting the system down, and it will carry out its own restoration process. In your system's case, this would apply ONLY if you were using the RAID1 style, and I expect you are NOT doing that.
On a RAID0 system failure, however, there is NO way to use the data on ONE drive to re-create the data missing from the other HDD unit that failed. Once one drive fails and is replaced, the remaining half data is useless. The complete set of ALL data must be restored from a separate data backup system. Thus, the "Hot Swap" feature is not really of any use to you when using the RAID0 configuration.
With that background, we get to your original query. If you were to replace ONE of your HDD units with an empty one even though there were NO problems with the original system, that new system would be just like a system that had one HDD unit failed and replaced. It would have NO usable information - you'd lose it all! So you could not possibly expand your data capacity this way.
Now, what IS possible is this. Once your system became nearly full, you could shut it down and remove BOTH of the HDD units in it. In doing so you MUST label each unit carefully about which slot it was in. This is because at some future time when you want to access their data again, those drive units MUST be re-installed in the SAME slots they came from. Having done that and stored them safely, you could then mount two brand new HDD units in the Pegasus2 case (bearing in mind the guidelines for compatible units) and have it create a new RAID0 array that is totally empty and ready for use. However, this does mean that you would not have immediate access to ANY of your old files - they would be on the disconnected drive units. If you wanted continued access to your old files PLUS new space, you would really need to buy and install a second storage device.
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