R900 Raid Controller

R900, meaning a Dell PowerEdge R900?

Yeah, your "friend" is probably just someone who looked at the spec sheet. The spec sheet, https://www.dell.com/downloads/global/products/pedge/en/pe_R900_spec_sheet.pdf -- top Google result for "R900" -- does say that the drive support is:

2.5” 6Gbs SAS (10K): 300GB, 600GB
2.5” SAS (10K rpm): 73GB, 146GB, 300GB
2.5” SAS (15K rpm): 36GB, 73GB
3.5” SAS (10K rpm): 300GB, 400GB, 600GB
3.5” Near-Line SAS (7.2K rpm): 500GB, 750GB, 1TB
3.5” SAS (15K rpm): 73GB, 146GB, 300GB, 450GB
3.5” 2.0TB SATA

which is very likely where that 2TB "limit" is coming from. In reality, the 2TB drive limit is probably just the largest SATA drive available at the time the spec sheet was written. There *is* -- sometimes -- a limit of 2TB with regards to a *partition* size. But that is a separate issue and only happens on drives with MBR which are limited to 2TB partitions. Still, a 4TB drive could have two 2TB partitions. The solution there is to use GPT, something that Linux can boot from but older versions of Windows (eg, 7) can't. With certain operating systems you would be, essentially, stuck having a smaller drive for the OS and then be able to have arbitrarily sized storage arrays, limited only by the number of SAS/SATA ports you have and the largest capacity drive available.

Looking at the spec sheet further for the RAID controller, there's a card Dell's called the PERC 6/IR. Running a Google search for that shows it was released prior to 2009 - which I believe pre-dates 3TB drives. This suggests that the 2TB specification is simply a place holder for "no known capacity limit" and is a sort of marketing term to let people know that it supports the largest available drives at the time of release, throwing doubt onto the other RAID cards which may show a 1TB capacity limit -- which again, was that card's marketing team's suggestion to show that it supports the largest available drives at its time of release.

Personally, I'd look up more about the PERC 6/IR, assuming that's the option you actually have, and see if there are any firmware updates for it and if there are, what they did. It's possible they did add an artificial 2TB limit and removed it in a later firmware, but this is really scummy. Another possibility is to find a newer PowerEdge server using that same RAID controller and see if it says the drive limit is 3TB, 4TB, 5TB, 6TB, 8TB, or 10TB by now. If it keeps going up then odds are there never was any limit at all and it was just marketing causing confusion in an attempt to avoid confusion.

In truth, there's very little technical reason for there to be a drive size limit. Numbers are commonly represented as either 32-bit or 64-bit values. With 32-bit the largest number you can create is 4294967295 (~4.2 billion) which would suggest a 4GB limit not a 4TB limit. With signed values, you can get half of that. With 64-bits, however, you'd be "limited" to 18,446,744,073,709,551,615, or something like 16.7 million TB -- needless to say we don't have to worry about this. Even a weird system using a 48-bit number would be limited to something like 262,000 TB (quarter million TB).