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Even if you could free up only 1GB on each of the drives, you could start the process with a RAID5 of 1GB per disk, migrate two TB of data into it, free up the 2GB in the old disks, to expand the RAID and rinse and repeat. It will take a very long time, and run a lot of risk due to increased stress on the old drives, but it is certainly something that’s theoretically achievable.
Not at all possible whatsoever though. If he has two drives nearly full, he would never be able to fit all replicable data on a RAID 5 of any kind.
What you're describing as a solution is the "3 jugs of water" problem. The difference is you need only one coherent set of data in order to even start a RAID array. Juggling between disks in this case would never make the solution OP is asking if all data can't fit on one single drive, due to the limitations of smallest drive capacity. You can't just swap things around and eventually come up with a viable array if ALL data can't be in one place at one time.
They’re going for RAID5, not 6, so with the third drive these’s no additional requirement.
Say for example if they have 2x 12T drive with 10T used each (they mentioned they’ve got 20T of data currently). They can acquire a 3rd 12T drive, create a RAID5 volume with 3x 1TB, thereby giving them 2TB of space on the RAID volume. They can then copy 2TB of data into the RAID volume, 1TB from each of the existing, verify the copy worked as intended, delete from outside, shrink FS outside on each of the drives by 1TB, add the newly available 1TB into the RAID, rebuild the array, and rinse and repeat.
At the very end, there’d be no data left outside and the RAID volume can be expanded to the full capacity available… assuming the older drives don’t fail during this high stress maneuver.
That is a clever aproach, and its just my caseuse, two 12 TB, about 19TB used.
And its for a personal project, so, i don't have any hurry.
Only for clarification several days could be 1 or 2 weeks or we are talking of more time?
I’m afraid I don’t have an answer for that.
It is heavily dependent on drive speed and number of times you’d need to repeat. Each time you copy data into the RAID, the array would need to write the data plus figuring out the parity data; then, when you expand the array, the array would need to be rebuilt, which takes more time again.
My only tangentially relatable experience with something similar scale is with raid expansion for my RAID6 (so two parity here compared to one on yours) from 5x8TB using 20 out of 24TB to 8x8TB. These are shucked white label WD red equivalents, so 5k RPM 256Mb cache SATA drives. Since it was a direct expansion, I didn’t need to do multiple passes of shrinking and expanding etc., but the expansion itself I think took my server a couple of days to rebuild.
Someone else mentioned you could potentially move some data into the third drive and start with a larger initial chunk… I think that could help reduce the number of passes you’d need to do as well, may be worth considering.