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The concrete problem:
I want to create a exact copy of a 4TB logical volume. (By exact I mean the RAID controller will create a copy.) Can I do this by simply using any old 4TB drive?

The more abstract question is:
Are two drives that claim two have the same volume (e.g. 1TB) actually of the exact same size (up to the last byte)? So assume I would like to do a byte by byte copy including potentially corrupted ones. Can I do this with any hard drive of the same "advertised" size? Or do they for some reason (i.e. manufacturer calculate volume differently) differ in size slightly?

EDIT: I assume this won't be the case as they might have (differently sized) bad segments?

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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.

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  • If I understand this correctly it should typically work to copy the "used" Volume of a drive byte by byte to a different one of the same size?
    – ls.
    Dec 3, 2020 at 15:59
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    Yes, unless on the original drive you allocated absolutely all the space and the new one is a few bytes less spacey.
    – Overmind
    Dec 4, 2020 at 9:24

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