Are two different hard drives the exact same size if they claim to have the same one (i.e. 1TB)?

Solution 1:

Two drives that claim two have the same volume (e.g. 1TB) should have the exact size of 1 000 000 000 000 (around 930GB) usable space. Manufacturers give a little more (a few extra sectors).

But physically, each drive has a few spare sectors that are reserved for reallocation events. So if they are not the same brand /model there may be small differences.

A 4TB logical volume may have exactly that decimal size, a little less or a little higher and may not fit into a 4TB drive.

If the volume is really 4TB (4 398 047 970 271 bytes) it will certainly not fit on a "4TB" drive which has 4 000 000 000 000 bytes. Most of the time, exact volumes are created with true binary size (4.0 TB or TiB as named now-days to make a difference), in which case you will need a larger drive (6TB) to fit that space in (which would be 4.4 decimal "TB").

So this is what you should remember: drive sizes are din decimal TB (aka fake TB), actual data is in binary format (true TB or TiB).

So if you RAID'ed 4 x 1 TB drives into a RAID 0 and formatted that as a single volume, it should generally fit into a 4TB drive, but it may also exceed it by just a few bytes.

If a volume of 4 real TB was created out of a larger portion of an array/disk, it will definitely not fit into a 4 TB drive.