OK, thanks. So if it makes no difference, why is there still a choice? backwards compatibility of some kind?

Guess I'm still not not understanding how/when you should use primary vs. non-primary, and doing some reading looks like you can/should mark one partition as "active" somehow too?

The basic thing is that only four primary partitions are allowed. One of these can be an Extended partition in which multiple logical partitions may be created. You can google to get more detailed info on the history of primary/extended/logical partitioning if you are interested.As far as the necessity, as indicated above it doesn't matter whether you use a primary or logical partition for Linux. AFAIK, you still need at least the boot files on a primary partition for a windows system to boot. A windows system AFAIK still needs to be have an active/bootable partition marked and this is not necessary for Linux

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OK. I guess I won't worry about it then. Thanks to everyone for your help on this, I'm going to mark it as closed.

"active" is also a non Linux (at least non Grub) concept.

I'm pretty sure Centos still uses some form of the Grub boot loader and very sure it still uses some boot loader that doesn't care about primary vs. logical and doesn't care about active.

A Windows boot loader needs to load part of itself from a primary partition. Windows compatible MBR code loads that part of the boot loader from the "active" primary partition. Windows then expects that its boot code was loaded from the active partition. Various multi-boot systems (including Grub) have ways of patching which partition is "active" at boot time in case you have more than one version of Windows or other non Linux OS that cares about "active". But all that only matters if you are multi-booting multiple OS's and some of them are not Linux (it only is complicated enough to need your attention when more than one of the OS's is non Linux).

Linux doesn't care about any of that.

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But sometimes the hardware does. My laptop (Linux only) refuses to boot if there is not one partition with the boot-flag. It doesn't matter at all which partition is marked, but the flag has to be there.