If I put a 2tb and 1 tb drive in and format as raid 0 (so 3tb raid) is this ok as there different capacities . Or do I have o use 2 x 1 or 2 tb drives?
You can build a RAID set (hardware or software) from mixed capacity disks.
However,
the usable capacity of any set created from them will be based on the smallest capacity disk (JBOD is not actually a RAID level, and technically speaking, level 0 isn't either, as it's not redundant).
So in your case of a 2TB + 1TB disk in a stripe set, the usable capacity is
2TB, not 3TB. You can gain access to that remaining 1TB by making a partition, but if you try to use it simultaneously with the RAID, you will see a performance hit.
Hope this helps.
Previous poster is correct.
Actually, it isn't. See above.
Most do use the same disks (identical model) in RAID configurations, particularly when starting out, but it's more common to see mixed sets in enterprise environments due to failed disks being replaced with something newer, or is in the middle of an upgrade (such as the set is undergoing Online Expansion or Online Migration).