Converting old Mac Pro to storage/server - storage? Etc.

Unless you expect to simultaneously run full tilt on multiple connections at once, that is not necessary. That's what a Gigabit Ethernet SWITCH is for. They are not expensive. I use two in my Home Network.

I just upgraded my Home MacOS Server to 20GB. So proud of that!

In response, it consistently uses less than FIVE, even with more available.

I have two 2009 MacPros working as servers in two departments. I replaced the SATA's with new drives, one for data, one for clone and one for time machine, you can backwards adapt a SATA 6, it will simply run at SATA 3 speed and I'm using CCCloner and TimeMachine

one is on 10.9, the other 10.10, the 10.9 build seems to be a bit more stable.

There is no practical limit on drive size. Anything you can buy will work fine. The real upper limit is more than 100,000 times larger than the largest drive you can buy.

Some users have reported problems in later versions of macOS where certain over 6GB drives would not Mount across a restart, but the work-around is just to do a Shutdown/Restart instead.

There’s no reason you can’t use larger drives (with one caveat). The ones I would avoid for use inside the Mac Pro are the so-called NAS drives, because of the not-mounting-after-restart issue. Toshiba 6TB drives work well in my 2010 Mac Pro, and WD Red drives are reported to work correctly also, in terms of mounting on restart. The problem for you may be the mounting sleds for the drives. I don’t know of any replacement sleds for a 2008 Mac Pro that would work with the screw position of some of the 6 TB drives. So 4 TB may be your practical upper limit for internal drives.

i agree with you, JimmyCMPIT

The real requirement is, that whatever your mounting scheme, it has to hold the drive straight enough to hit the backplane connector and seat properly, and keep the drive from falling out. Not much else.

When SSD drives first come out, I could not get one onto a Mac Pro sled, so I invented a way using one CD-mounting screw and some double-stick tape. it was fussy to install, but worked just fine.

To run at Gigabit Ethernet speeds, you must use cables with all 8 conductors present.

I have an 8TB iron wolf in one of my bays, it's not entirely aligned with the sled for the screws but it works fine.